ASTM F2161-21

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: F2161 − 21
Standard Guide for
Instrument and Precision Bearing Lubricants—Part 1 Oils
This standard is issued under the fixed designation F2161; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope without replenishment. The lubricant must be able to support
high loads but cannot be so viscous that it will interfere with
1.1 This guide is a tool to aid in the choice of an oil for
the operation of the bearing at very high speeds or low
precision rolling element bearing applications. There are two
temperatures, or both. The lubricant must provide boundary
areas where this guide should have the greatest impact: (1)
lubrication during low-speed or intermittent operation of the
when a lubricant is being chosen for a new bearing application
bearing.And, in many applications, its vapor pressure must be
and (2) when a lubricant for a bearing has to be replaced
low enough under operating conditions that evaporative losses
because the original lubricant specified for the bearing can no
do not lead to lubricant depletion or contamination of nearby
longer be obtained.The Report (Section 5) contains a series of
components.Theseandotherconsiderationsdictatedtheseries
testsperformedbythesamelaboratoryonawidevarietyofoils
of tests that were performed on each lubricant included in this
commonly used in bearing applications to allow comparisons
study.
of those properties of the oil that the committee thought to be
most important when making a choice of lubricant. This guide 1.3 Another important consideration was encompassed in
containsalistingofthepropertiesofoilsbychemicaltype,that thisstudy.Almostallofthetestingwasperformedbythesame
is, ester, silicone, and so forth. This organization is necessary laboratory, The Petroleum Products Research Department of
since the operational requirements in a particular bearing the Southwest Research Institute in SanAntonio, Texas, using
application may limit the choice of lubricant to a particular ASTM procedures. This continuity of testing should form a
chemical type due to its temperature stability, viscosity index solid basis for comparing the properties of the multitude of
ortemperature-vaporpressurecharacteristics,andsoforth.The lubricantstestedbyavoidingsomeofthevariabilityintroduced
Report includes the results of tests on the oils included in this when lubricants are tested by different laboratories using
study. The Report recommends replacement lubricants for different or even the “same” procedures.
those oils tested that are no longer available. The Report also
1.4 It should be noted that no functional tests (that is,
includes a glossary of terms used in describing/discussing the
bearingtests)wereperformed.Theresultsofthefour-ballwear
lubrication of precision and instrument bearings. The Report
test give some comparison, “a figure of merit,” of the lubrica-
presents a discussion of elastohydrodynamic lubrication as
tion properties of the oils under the condition of this test. But
applied to rolling element bearings.
experience has shown that testing the lubricant in running
1.2 Although other compendia of lubricant properties have bearings is the best means of determining lubricant perfor-
been published, for example, the Barden Product Standard, mance.
Lubricants and the NASA Lubricant Handbook for the Space
1.5 This international standard was developed in accor-
Industry , none have centered their attention on lubricants
dance with internationally recognized principles on standard-
commonly used in precision rolling element bearings (PREB).
ization established in the Decision on Principles for the
The PREB put a host of unique requirements upon a lubricant.
Development of International Standards, Guides and Recom-
The lubricant must operate at both high and low temperatures.
mendations issued by the World Trade Organization Technical
Thelubricantmustprovidelubricationformonths,ifnotyears,
Barriers to Trade (TBT) Committee.
2. Referenced Documents
This guide is under the jurisdiction of ASTM Committee F34 on Rolling
2.1 ASTM Standards:
Element Bearings and is the direct responsibility of Subcommittee F34.02 on
Tribology and was developed by DoD Instrument BearingWorking Group (IBWG)
D92Test Method for Flash and Fire Points by Cleveland
former F34.
Open Cup Tester
Current edition approved Jan. 15, 2021. Published January 2021 originally
approved in 2001. Last previous edition approved in 2010 as F2161–10, which was
withdrawn July 2020 and reinstated in January 2021. DOI: 10.1520/F2161-21.
2 4
Product Standard, Lubricants,availablefromTheBardenCorp.,Danbury,CT. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
NASA Lubricant Handbook for the Space Industry, Ernest L. McMurtrey , contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
NASA Technical Memorandum TM-86556,GeorgeC.MarshallSpaceFlightCenter, Standards volume information, refer to the standard’s Document Summary page on
National Aeronautics and Space Administration, December 1985. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F2161 − 21
D97Test Method for Pour Point of Petroleum Products temperature, T. As the temperature increases viscosity de-
D445Test Method for Kinematic Viscosity of Transparent creases. ASTM International has adopted the following rela-
andOpaqueLiquids(andCalculationofDynamicViscos-
tionship between kinematic viscosity and temperature:
ity)
log log ν10.8 5 m log T1c
~ !
10 10 10
D974Test Method for Acid and Base Number by Color-
where:
Indicator Titration
D972Test Method for Evaporation Loss of Lubricating
m and c = constants for each fluid.
Greases and Oils
ASTM International supplies chart paper with the ordinate
D1331Test Methods for Surface and Interfacial Tension of
proportional to log log (ν + 0.8) and with the abscissa
10 10
Solutions of Paints, Solvents, Solutions of Surface-Active
proportional to log T. Thus the values of kinematic viscosity
Agents, and Related Materials
versustemperaturecanbeplottedasastraightlineonthepaper
D2270Practice for Calculating Viscosity Index from Kine-
allowingextrapolationofvaluesintermediatetothosethathave
matic Viscosity at 40°C and 100°C
been measured.
D4172Test Method for Wear Preventive Characteristics of
Absolute viscosity is a weak function of the pressure
Lubricating Fluid (Four-Ball Method)
imposed upon the fluid. However, the pressures generated in
2.2 Government Documents :
the ball-race contact zone of a ball bearing can be on the order
MIL-DTL-53131Lubricating Oil, Precision Rolling Ele- 3 5
of10 GPa(10 psi)andatthesepressuressignificantincreases
ment Bearing, Plolyalphaolefin Based
in viscosity can occur. Experiments have shown that viscosity
MIL-L-6085Lubricating Oil,Aircraft Turbine Engine, Syn-
varies exponentially with pressure and can be expressed as
thetic Base
follows:
MIL-L-14107Lubricating Oil, Weapons, Low Temperature
η 5 η exp αp
~ !
MIL-L-23699Lubricating Oil, Aircraft Turbine Engines,
Synthetic Base
where:
MIL-L-7808Lubricating Oil,Aircraft Turbine Engine, Syn-
η = viscosity at a pressure of one atmosphere,
thetic Base
p = pressure, and
MIL-L-81846Lubricating Oil, Instrument, Ball Bearing,
α = pressure-viscosity coefficient.
High Flash Point
A table of values of α for some common classes of bearing
MIL-S-81087Silicone, Fluid, Chlorinated Phenyl Methyl
lubricants can be found after the definition of pressure-
Polysiloxane
viscosity coefficient included in this glossary.
Recent work has shown that the viscosity changes with
3. Terminology
temperature can also be modeled by an exponential relation-
3.1 Definitions of Terms Specific to This Standard:
ship. Thus, viscosity at any pressure and temperature can be
3.1.1 ABEC, n—Annular Bearing Engineer’s Committee of
expressed as follows:
the American Bearing Manufacturers Association (ABMA).
η 5 η exp αp1β 1/T11/T
~ ~ !!
The ABEC establishes bearing tolerance classes. Precision
T, p 0 0
bearings are ABEC 5P and ABEC-5T and higher.
where:
3.1.2 absolute viscosity (η), n—(sometimes called dynamic
β = temperature-viscosity coefficient.
viscosity or just viscosity)—a measure of the tendency of the
3.1.3 acid number, n—a measure of the quality of a lubri-
fluid to resist shear. The elastohydrodynamic theory (EHD)
cant.Highacidnumbers(muchhigherthanthefreshoil)arean
film thickness and torque losses in a ball bearing are very
indication of lubricant oxidation/degradation. Oils with high
strong functions of η. Since the ratio of absolute viscosity to
acid numbers should not be used.Acid number is measured as
density, η/ρ, appears frequently in hydrodynamic analyses, it
milligrams of KOH needed to neutralize one gram of oil.
wasgivenitsownname,kinematicviscosity,ν.Thecgsunitof
viscosity is the centipoise (cP). The SI unit of viscosity is the
3.1.4 additive, n—any chemical compound added to a lu-
Pascal-s (Pa-s).
bricant to improve or meet special needs necessary for service
Absolute viscosity is defined for a Newtonian fluid as
(formulated lubricants). The most important additives are
follows.Theshearstressatanypointinthefluidisproportional
antioxidants, rust and corrosion inhibitors, and extreme pres-
to the rate of shear. The proportionality constant is called the
sure (EP) and antiwear (AW) additives.
absolute viscosity. Viscosity is thus defined by the force, F,to
3.1.5 antioxidants (oxidation inhibitors),n—chemical com-
move one surface of area, A, with respect to another surface
pounds used to improve the oxidation stability and subsequent
separatedbyafluidfilm,h,ataspeed,U,throughthefollowing
deterioration of lubricants.
relationship:
3.1.6 boundary lubrication, n—a condition of lubrication in
η 5 F/A h/U
~ !~ !
which the friction between two surfaces in relative motion is
The value of the absolute viscosity changes greatly with
determined by the roughness of the surfaces and by the
properties of the lubricant other than viscosity. Antiwear and
5 extreme pressure additives reduce the wear of components
Available from Document Automation and Production Service, Building 4/D,
700 Robins Ave., Philadelphia, PA 19111–5094. operating under this regime.
F2161 − 21
3.1.7 centipoise, n—a unit of dynamic viscosity.The unit in tion and also to an alteration of the fluid properties of a
the cgs system is one centipoise (cP). The SI unit of dynamic lubricant (especially an increase in the viscosity of blended
viscosity is 1 Pa-s and equivalent to 10 cP. lubricants). The evaporation loss is expressed as a weight loss
-6
in milligrams (10 kg) or wt%.
3.1.8 centistoke, n—a unit of kinematic viscosity. The unit
in the cgs system is one centistoke (cSt). The SI unit of 3.1.20 fire point, n—the lowest temperature at which the
2 6
kinematic viscosity is 1 m /s and is equivalent to 10 cSt.
vapor or a lubrication fluid ignites under specified test condi-
tions and continues to burn for at least 5 s without the benefit
3.1.9 compatibility, n—a measure of the ability of a lubri-
of an outside flame. The fire point is a temperature above the
cant to be mixed with other lubricants or bearing preservatives
flash point. Perfluoropolyethers have no fire point.
(fluids that form films on metal surfaces to prevent corrosion
duringstorage)toformauniformmixturewithoutcausingany
3.1.21 flash point, n—the lowest temperature of a lubrica-
resultant reaction or precipitation of material. Compatibility is
tion fluid at which the fluid gives off vapors that will ignite
also a measure of the ability of a lubricant not to cause any
when a small flame is periodically passed over the liquid
detrimental effect to metal, plastic, or elastomer materials.
surfaceunderspecifiedtestconditions.Theflashandfirepoints
3.1.9.1 Discussion—It is recommended that any preserva-
provide a rough characterization of the flammable nature of
tive material be removed from bearings before lubrication. lubrication fluids. Perfluoropolyethers have no flash point.
3.1.10 contamination, n—(1) The presence of mostly solid
3.1.22 four-ball tester, n—a tester used to evaluate the wear
foreignmaterialslikesand,grindingpowder,dust,andsoforth,
behavior of lubricants under extreme pressure. Four steel balls
in a lubricant that might cause an increase in wear, torque, and
are arranged in a pyramidal shape. During the test, the three
noise and result in reduced bearing life. (2) The presence of
balls comprising the base of the pyramid are stationary while
fluids like water, solvents, and other oils that might cause
theupperballrotates.Thelubricantsampleisplacedintheball
accelerated oxidation, washout, rusting, or crystallization of
pot. The average wear scar (measured in millimetres) formed
theadditivesandotherphenomenathatreduceabearing’slife.
on the stationary balls is reported.
3.1.11 corrosion, n—the gradual destruction of a metal
3.1.23 fretting corrosion, n—a special type of wear pro-
surfaceduetochemicalattackcausedbypolaroracidicagents
duced on materials in intimate contact that are subjected to the
like humidity (water), compounds formed by lubricant
combined action of oscillatory motions of small amplitudes
deterioration, or by contaminants from the environment.
and high frequencies. Fretting corrosion appears similar to
atmospheric corrosion (rust) as a reddish-brown layer on steel
3.1.12 corrosion inhibitors, n—corrosion inhibitors protect
surfaces.
metalsurfacesagainstcorrosionorrustbyformingaprotective
coating or by deactivation of corrosive compounds formed
3.1.24 interfacial tension, n—when two immiscible liquids
during the operation of a bearing.
are in contact, their interface has many characteristics in
common with a gas-liquid surface. This interface possesses
3.1.13 density, n—the mass per unit volume of a substance.
interfacial free energy because of the unbalanced attractive
Thecgsunitofdensity(ρ)is1g/cm ,andtheSIunitofdensity
forces exerted on the molecules at the interface by the
is 1 kg/m . Density depends on the chemical composition and
moleculeswithintheseparatephases.Thisfreeenergyiscalled
in itself is no criterion of quality. It is a weak function of
the interfacial tension.
temperature and pressure for liquids and solids.
3.1.25 instrument bearings, n—all bearings whose outer
3.1.14 DN value, n—the product of the bearing bore diam-
diameteris30mmorless,asdefinedbyTheAmericanBearing
eter in millimetres multiplied by the speed in revolutions per
Manufacturers Association (ABMA).
minute (compare to nD -value).
m
3.1.26 kinematic viscosity, n—theratioofabsoluteviscosity
3.1.15 dynamic viscosity, n—another name for absolute
to fluid density. This ratio arises frequently in lubrication
viscosity.
analyses and thus kinematic viscosity has become a separate
3.1.16 elastohydrodynamic theory (EHD), n—See Appen-
term describing the viscosity of a fluid. Many experimental
dix X1.
measurements of viscosity of fluids result in a measure of
3.1.17 EP lubricants (extreme pressure lubricants),
kinematic viscosity from which absolute viscosity is calcu-
n—lubricants (oil or greases) that contain extreme pressure
lated. See absolute viscosity. The cgs unit of kinematic
additives to protect the bearings against wear and welding
viscosity is cSt, and the SI unit is m /s. The viscosity of a
(scoring).
PREB oil is a major factor in lubricant selection.The viscosity
is directly involved in frictional, thermal, and fluid film
3.1.18 esters, n—esters are formed from the reaction of
acids and alcohols. Esters form a class of synthetic lubricants. conditions which reflect the influence of load, speed,
Esters of higher alcohols with divalent fatty acids form diester temperature, and design characteristics of the bearing being
lubricants while esters of polyhydric alcohols are called the lubricated.
polyolesterlubricants.Theselatterestershavehigherviscosity
3.1.27 military(MIL)specifications,n—specificationsofthe
and are more heat-resistant than diesters.
U.S. Armed Forces indicating the minimum mandatory re-
3.1.19 evaporation loss, n—lubrication fluid losses occur- quirements for an item that is to be procured. Military
ring at higher temperatures or under vacuum, or both, due to specifications are widely used as procurement requirements
evaporation.Thiscanleadtoanincreaseinlubricantconsump- and as a quality standard.
F2161 − 21
3.1.28 mineraloil,n—oilsbasedonpetroleumstocks.These 3.1.32 precision bearings, n—regardlessofsize,theclassof
oils come in two types, naphthenic and paraffinic. The naph- bearings used in instrument types of applications and with
thenic oils contain unsaturated hydrocarbons, usually in the similar tolerances as instrument bearings. Bearings usually
form of aromatic species. The paraffinic oils are primarily used in these applications are of high quality and are not put
saturated hydrocarbons with only low levels of unsaturation. under high stress, thus they usually do not fail under fatigue.
3.1.29 nD -value (index), n—also called speed index—a 3.1.33 perfluoropolyethers (PFPE or PFAE), n—fully fluo-
m
relative indicator of the lubricant stress imposed by a bearing rinated long-chain aliphatic ethers. The perfluoropolyethers
rotating at a given speed, where n is the rotational speed of the show some extraordinary properties like chemical inertness,
rollingelementbearinginrevolutionsperminuteand D isthe nonflammability, high thermal and oxidative resistance, very
m
mean diameter in millimetres (arithmetic mean of bore diam- good viscosity-temperature characteristics, and compatibility
eter d and outside diameter D). The speed index is multiplied with a wide range of materials, including metals and plastics.
by a factor k depending on the roller element bearing type: Theperfluoropolyethers,however,arenotsuitableforusewith
a
aluminum, magnesium, and titanium alloys. The perfluoropo-
k = 1 for deep groove ball bearings, angular contact ball bearings,
a
self-aligning ball bearings, radially loaded cylindrical roller bearings,
lyethers are not compatible with other types of synthetic fluids
and thrust ball bearings,
and mineral oils and cannot dissolve common lubricant addi-
k = 2 for spherical roller bearings, taper roller bearings, and needle
a
tives.
roller bearings, and
k = 3 for axially loaded cylindrical roller bearings and full
a
3.1.34 pH value, n—a scale for measuring the acidity or
complement roller bearings.
alkalinity of a product. Zero pH is very acid, 7 is neutral, and
The factor k takes into account the various rates of slid-
a
14 is very alkaline.
ing friction that usually occurs during the operation of a roll-
3.1.35 poise (P), n—See centipoise (cP).
ing element bearing. The nD -value is an aid in choosing a
m
suitable lubricant viscosity for a given bearing speed and is 3.1.36 pour point, n—(of a lubricating fluid)—the lowest
temperature at which the lubricating fluid will pour, or flow.
particularly applicable to grease-lubricated bearings.
3.1.30 neutralization number, n—ameasureoftheacidityor 3.1.37 pressure-viscosity coeffıcient, n—thedynamicviscos-
ity of a fluid increases with increasing pressure. The depen-
alkalinity of a lubricating fluid. The test determines the
quantity of base (milligrams of potassium hydroxide) or acid dence of viscosity (absolute), η, on pressure, p, can be
expressed by the equation:
(also expressed as milligrams of potassium hydroxide) needed
to neutralize the acidic or alkaline compounds present in a
η 5 η exp αp
~ !
lubricatingfluid.Actually,theneutralizationnumberisnotone
where:
number but several numbers: strong acid number, total acid
η = absolute viscosity at pressure, p,
number, strong base number, and total base number. The
η = absolute viscosity at one atmosphere, and
neutralization number is used for quality control, and to
α = the pressure-viscosity coefficient.
determine changes that occur in a lubricant in service.
The pressure-viscosity coefficient is very small and varies
3.1.31 oxidation stability, n—the stability of a lubricant in
with the chemical composition of the fluid. Some values of α
the presence of air or oxygen is an important chemical
for the classes of lubricants discussed in the Report section are
property. Oxidation stability has a strong influence on numer-
given in Table 1.
ousphysicalpropertiesofalubricant.Thesepropertiesinclude
One limitation of the use of η and the corresponding
thechangeofviscosityunderstaticconditionsforlongperiods 0
equation is that the measurements of η are made under static
of time (storage) or when exposed to temperatures high above
conditions where the pressure is held constant while the
roomtemperature,orboth.Theslowchemicalreactionoffluid
viscosity attains a steady-state value. In actual bearing
(base oil) and oxygen (air) is called oxidation. Inhibitors (see
operations, the lubricant may see high pressure in the contact
antioxidants) are used to improve the oxidation stability of the
zoneforonlyafewmillisecondsandtheviscositychangesdue
lubricants. Synthetic fluids, especially perfluoropolyethers and
to this high pressure may not reach steady-state values.
silicones, are much more resistant to oxidation than mineral
oils.
3.1.38 rated viscosity, (ν ), n—the kinematic viscosity at-
tributed to a defined lubricating condition of a rolling element
bearing. The rated viscosity is a function of the speed and can
TABLE 1 Pressure-Viscosity Coefficients for the Lubricant
be determined by the mean bearing diameter in millimetres
Classes Covered in This Guide
-3
(10 m) and the rotational speed (rpm). More details can be
A
α
Oil Type
-1
(GPa ) found in Appendix X1.
Mineral oil (paraffinic-naphthenic) 21
3.1.39 repeatability, n—a criterion for judging the accept-
Mineral oil (naphthenic-aromatic) 30
ability of test results. Repeatability is the difference between
Polyalphaolefin 18
successive test results obtained by the same operator with the
Diester-(2-ethylhexyladipate) 7.6
Polyolester (pentaerythritolvalerate) 7.5
same apparatus under constant operating conditions on identi-
Polydimethylsilicone (1000 mm /s at 40 °C) 2.3
caltestmaterial.Repeatabilityisusuallyreportedasarangeof
PFPE–linear 4–12
PFPE–branched 26–36
A
1 atm = 0.001013 GPa.
Journal of Synthetic Lubrication , Vol 1, 1984, pp. 73-86.
F2161 − 21
values that would, in the normal and correct operation of the a liquid spontaneously contracts to the smallest possible area.
test method, encompass two standard deviations from the For example, liquid droplets assume a spherical shape if no
median value of the test. outside forces are acting on the droplet. To deform the droplet
from its spherical shape, a definite amount of work must be
3.1.40 reproducibility, n—a criterion for judging the accept-
done. This work (or energy expended) when normalized per
abilityoftestresults.Reproducibilityisthedifferencebetween
unit area is called surface tension and has the unit mN/m
two single and independent results, obtained by different
(formally dyne/cm). Surface tension is dependent upon tem-
operators working with identical test material. This difference,
perature but not upon pressure. The surface tension is a
in the long run and under normal and correct operation of the
measure for the wetting of a bearing surface and for the
test method, would not exceed a specified value.
creeping (spreading) property of a lubricant. Fluids with low
3.1.41 saponification number, n—a measure of the amount
surfacetensionslikedimethylsiliconesshowimprovedwetting
of constituents of a lubrication fluid that will easily saponify
but increased creeping (migration) tendency.
under test conditions. The saponification number is expressed
3.1.49 swelling properties, n—theswellingofnaturalrubber
in milligrams of potassium hydroxide that are required to
and elastomers under the influence of lubricants.
neutralize the free and bonded acids contained in one gram of
3.1.50 synthetic fluids, n—lubricating fluids produced by
lubricating fluid. The saponification number is a measure of
chemicalsynthesis.Thesyntheticroutetolubricantsallowsthe
fatty acids compounded in an oil and a measure of the state of
manufacturer to introduce those chemical structures into the
oil deterioration.
lubricant molecule that will impart specific properties into the
3.1.42 saponify, v—to hydrolyze an ester and to convert the
resultant fluid such as very low pour point, good viscosity-
free acid into soap.
temperaturerelationship,lowevaporationloss,longlubricating
3.1.43 seal compatibility, n—the extent of the reaction of
lifetime, and so forth.
sealing materials with lubricating oils, greases, and other
3.1.51 viscosity, n—See absolute viscosity.
fluids. The reaction can result in swelling, shrinking,
3.1.52 viscosity index (VI), n—indicatestherangeofchange
plasticizing, embrittlement, or even dissolution. Operating
in viscosity of a lubricating fluid within a given temperature
temperatures and lubricant composition are dominant factors
range. With an increase in the viscosity index, the fluid
influencing the extent of the interaction between the sealing
becomes less sensitive to temperature, that is, a low-viscosity
material and the lubricating fluid.
index signifies a relatively large change, whereas a high-
3.1.44 setting point, n—of a lubricating fluid—the tempera-
viscosity index relates to a relatively small change in viscosity
ture at which the fluid ceases to flow when cooled under
with temperature.
specifiedconditions.Thelow-temperaturebehaviorofthefluid
3.1.53 wear, n—the attrition or rubbing away of the surface
slightly above the setting point may be unsatisfactory and,
of material as a result of mechanical action.
therefore,thisbehaviorshouldbedeterminedbymeasuringthe
low-temperature kinematic or absolute viscosity.
4. Significance and Use
3.1.45 shelf life, n—theexpressionshelflifeofalubricantis
4.1 The purpose of this guide is to report on the testing of,
not exactly specified. Two versions of the definition exist:
to discuss and compare the properties of, and to provide
(1) shelf life—the ability of a lubricated part to function
guidelines for the choice of lubricants for precision rolling
even after long-term storage. This definition is very critical
element bearings (PREB). The PREB are, for the purposes of
because it includes not only the aging properties of the
this guide, meant to include bearings of ABEC 5 quality and
lubricant used but also the loss of lubricant due to evaporation
above. This guide limits its scope to oils used in PREB and is
and creeping.
to be followed by a similar document to encompass greases
(2) shelf life—the storage stability of the bulk lubricant in
used in PREB.
its original container. Stability is defined here as no change in
the physical or chemical properties of the lubricant. 4.2 The number of lubricants, both oils and greases, used in
PREB increased dramatically from the early 1940s to the mid
3.1.46 siliconeoils,n—syntheticfluidscomposedoforganic
1990s.Inthebeginningofthisperiod,petroleumproductswere
esters of long chain complex silicic acids. Silicone oils have
the only widely available base stocks. Later, synthetic lubri-
better physical properties than mineral oils. However, silicone
cants became available including synthetic hydrocarbons,
oils have poorer lubrication properties, lower load-carrying
esters, silicones, and fluorinated materials, including perfluo-
capacity, and a strong tendency to spread on surfaces (see
rinated ethers and the fluorosilicones. This broad spectrum of
surface tension). To prevent this spreading the use of barrier
lubricant choices has led to the use of a large number of
films is necessary.
different lubricants in PREB applications. The U.S. Depart-
3.1.47 stability, n—the resistance of a lubricant to a change
ment of Defense, as a user of many PREB, has seen a
in its properties after being stored for a defined period of time.
significant increase in the logistics effort required to support
The methods to test a lubricant for stability are defined in
the procurement and distribution of these items. In addition, as
individual military or commercial specifications.
time has passed some of the lubricants used in certain PREB
3.1.48 surface energy/surface tension, n—a fundamental are no longer available.The SRG Series, LSO-26, andTeresso
p
...