Lubrication and Reliability Handbook

254 Pages·2000·13.97 MB·English

by  M.J. Neale (Eds.)

INTRODUCTION
This handbook is intended to help engineers in industry filters and coolers as well as the interconnecting piping
with the operation and maintenance of machinery. It systems and the necessary instrumentation and warning
gives the information that these engineers need in a devices.
form that is instantly accessible and easy to read.
The manufacturers of machinery provide guidance on
Machine operation
the operation, lubrication and maintenance required for
their particular machines. However, there are, of course, The machine manufacturers and/or process designers
many different machines in an industrial plant or service will usually provide the necessary guidance on machine
organisation, supplied by various manufacturers, and operating conditions. The operating engineers can
there is a need to select as many similar lubricants as however benefit from additional guidance on running in
possible, and to use related maintenance techniques. procedures, and on lubricant related operating prob-
This book attempts to bridge the gap which exists lems, such as potential lubricant deterioration due to
between the available data on the various machines, by high or low temperatures, and the effect of contaminant
providing overall guidance on how to co-ordinate the process gases and liquids. Information is provided on
recommendations of the various manufacturers. these areas, together with data on fire or health hazards
The handbook is structured in a number of sections to from lubricants.
make it easier to use, and to bring together related
subjects, so that the reader when focusing on a particular
Machine maintenance
problem can also refer to related material that is likely to
be of interest. The various sections are listed here in this To keep the machines in a plant or fleet operating
introduction, to provide some overall guidance, addi- effectively, requires good maintenance procedures. The
tional to that available in the contents list and the handbook reviews the suitability of the various main-
index. tenance methods for various types of machines and gives
guidance on their selection. Condition based main-
tenance is covered in detail with the various methods by
Lubricants
which the condition of a machine can be monitored
while it is in operation, so that future essential main-
This section describes the various types of lubricant that
tenance can be planned. Such methods include tem-
are available with guidance on their overall properties
perature measurement, vibration analysis, wear debris
and performance. Detailed information is provided on
analysis, and lubricant tests, as well as methods of
mineral oils, synthetic oils, greases and solid lubricants,
assessing the operating performance of machine
as well as on the various oil additives that are commonly
components.
used. Since some machines are now lubricated by their
own process fluids information is also given on the
viscosity of water, refrigerants and various hydrocarbons Component failure
and chemicals.
When a failure does occur on one of the working
components of a machine, such as a bearing, gear, seal or
Lubrication of components coupling, it is useful to have guidance on understanding
the causes of the failure from the appearance of the
The lubrication of machines relates to the lubrication of
failed component. This section therefore includes a
their various moving components. This section gives
large number of photographs of machine components
guidance on the selection of lubricants to match the
showing the typical surface appearance associated with
needs of the components under a range of operating
the various failure modes.
conditions. The components covered are plain and
rolling bearings, gears, roller chains, wire ropes, flexible
Component repair
couplings and slides.
Finally, after a failure has occurred it is useful to have
Lubrication systems guidance on how a worn surface can be rebuilt or
refaced, or how a bearing or friction surface can be
The next subject requiring review is the optimum relined.
method of feeding the lubricant to the various machines This handbook is based on experience from around
and their components. This can range from manual the world, over many years, of the investigation of
greasing to automated centralising greasing systems, and problems with machines of all kinds, and of dealing with
from splash, wick and ring oil feeding to pressurised mist these by practical and economical solutions. It is hoped
systems and full size oil circulation systems. Detailed that it will be helpful to the many engineers involved in
guidance is also given on the selection and design of machine operation and maintenance of all kinds of
circulation system components such as oil tanks, pumps, machinery and plant.
CONTRIBUTORS
Section Author
Selection of lubricant type .A .R Lansdown MSc, PhD, FRIC, FlnstPet
Mineral oils .T .I Fowle BSc(Hons), ACGI, CEng, FIMechE
Synthetic oils .A .R Lansdown MSc, PhD, FRIC, FInstPet
Greases N. Robinson & .A .R Lansdown MSc, PhD, FRIC, FInstPet
Solid lubricants and coatings J. .K Lancaster PhD, DSc, FInstP
Other liquids .D .T Jamieson FRIC
Plain bearing lubrication J. c. Bell BSc, PhD
Rolling bearing lubrication .E .L Padmore CEng, MIMechE
Gear and roller chain lubrication J. Bathgate BSc, CEng, MIMechE
Wire rope lubrication D. .M Sharp
Lubrication of flexible couplings J. D. Summers-Smith BSc, PhD, CEng, FIMechE
Slide lubrication M.J. Neale OBE, BSc(Eng), DIC, FCGI, WhSch, FEng, FIMechE
Lubricant selection .R .S Burton
Selection of lubrication systems .W J. J. Crump BSc, ACGI, FInstP
Total loss grease systems .P .L Langborne ,AB CEng, MIMechE
Total loss oil and fluid grease systems E G. E Seldon CEng, MIMechE
Mist systems .R .E Knight BSc, FCGI
Dip splash systems J. Bathgate BSc, CEng, MIMechE
Circulation systems .D .R Parkinson FInstPet
Design of oil tanks .A G. .R Thomson BSc(Eng), CEng, AFRAeS
Selection of oil pumps A.j. Twidale
Selection of filters and centrifuges .R H. Lowres CEng, MIMechE, MIProdE, MIMarE, MSAE,
MBIM
Selection of heaters and coolers J. H. Gilbertson CEng, MIMechE, AMIMarE
A guide to piping design .P D. Swales BSc, PhD, CEng, MIMechE
Selection of warning and protection devices A.J. Twidale
Commissioning lubrication systems N. .R .W Morris
Running-in procedures .W .C Pike BSc, ACGI, CEng, MIMechE
Industrial plant environmental data .R .L .G Keith cSB
High pressure and vacuum .A .R Lansdown MSc, PhD, FRIC, FInstPet
J. D. Summers-Smith BSc, PhD, CEng, FIMechE
High and low temperatures M.J. Todd MA
Chemical effects H. H. Anderson BSc(Hons), CEng, FIMechE
Maintenance methods M.J. Neale OBE, BSc(Eng), DIC, FCGI, WhSch, FEng, FIMechE
Condition monitoring M.J. Neale OBE, BSc(Eng), DIC, FCGI, WhSch, FEng, FIMechE
Operating temperature limits J. D. Summers-Smith BSc, PhD, CEng, FIMechE
Vibration analysis M.J. Neale OBE, BSc(Eng), DIC, FCGI, WhSch, FEng, FIMechE
Wear debris analysis .M H. Jones BSc(Hons), CEng, MIMechE, MInstNDT
M.J. Neale OBE, BSc(Eng), DIC, FCGI, WhSch, FEng, FIMechE
Lubricant change periods and tests J. D. Summers-Smith BSc, PhD, CEng, FIMechE
Lubricant biological deterioration .E .C Hill MSc, FInstPet
Component performance analysis M.J. Neale OBE, BSc(Eng), DIC, FCGI, WhSch, FEng, FIMechE
Allowable wear limits H. H. Heath FIMechE
CONTRIBUTORS
Section Author
Failure patterns and failure analysis j. .D Summers-Smith BSc, PhD, CEng, FIMechE
M.J. Neale OBE, BSc(Eng), DIC, FCGI, WhSch, FEng, FIMechE
Plain bearing failures E .T Holingan BSc(Tech), MIF
Rolling bearing failures .W J. J. Crump BSc, ACGI, FInstP
Gear failures .T .I Fowle BSc(Hons), ACGI, CEng, FIMechE
H.J. Watson BSc(Eng), CEng, MIMechE
Piston and ring failures M.J. Neale OBE, BSc(Eng), DIC, FCGI, WhSch, FEng, FIMechE
Seal failures .B .S Nau BSc, PhD, ARCS, CEng, FIMechE, MemASME
Brake and clutch failures .T .P Newcombe DSc, CEng, FIMechE, FInstP
.R .T Spurr BSc, PhD
Wire rope failures .S waM ,AM CEng, MIMechE
Fretting of surfaces .R .B Waterhouse ,AM PhD, MIF
Wear mechanisms .K H. .R Wright PhD, FInstP
Repair of worn surfaces .G .R Bell BSc, ARSM, CEng, FIM, FWeldI, FRIC
Wear resistant materials H. Hocke CEng, MIMechE, FIPlantE, MIMH, FIL
.M Bartle CEng, MIM, DipIM, MIIM, AMWeldI
Repair of plain bearings .P .T Holligan cSB (Tech), MIF
Repair of friction surfaces .T E Newcomb DSc, CEng, FIMechE, FInstP
.R .T Spurr BSc, PhD
Viscosity of lubricants H. Naylor BSc, PhD, CEng, FIMechE
Surface hardness M.J. Neale OBE, BSc(Eng), DIC, FCGI, WhSch, FEng, FIMechE
Surface finish and shape .R .E Reason DSc, ARCS, SRF
Shape tolerances of components J.J. Crabtree BSc(Tech) Hons
S.I. units and conversion factors M.J. Neale OBE, BSc(Eng), DIC, FCGI, WhSch, FEng, FIMechE
Selection of lubricant type 1A
elbaT 1.1 Importance of lubricant properties in relation to bearing type
~ epyT o f tnenopmoc nialP lanwoj nepO ,sraeg kcolC dna ,segniH .sedils
tnacirbuL ytreporp ~ gniraeb gnilioR gniraeb Closed&,,. ,,tpo, c~, .~, ~ut t~o, t,~,. .~,
.1 Boundarylubricating properties + ++ +++ ++ ++ +
2. Cooling + + + + +++
3. Friction or torque + + + ++ -- ++ +
4. Ability to remain in bearing + + + + +++ +
5. Ability to seal out contaminants -- + + + -- +
6. Temperature range + + + ++ + -- +
7. Protection against corrosion + + + ++ -- +
8. Volatility + + ++ ++ +
Note: The relative importance of each lubricant property in a particular class of component is indicated on a scale from + + + =
highly important to - --- quite unimportant.
Speed, ft/min
2 10 100 1000 10000 100000
1000000
100000
(cid:12)9
100000
10000
l
_ - .
- ",,~. ~."...- ;!N~,, ~,~,. -
1
10000 ~ . . . . ~/~,,,. ~ ~------~ \ oo
E
z Limit rof esaerg ~ 1000 .- i/I
- Ii
".0 6.
:"'"7:::::".:.'.:'.:';;.:"
::-: ".' :-.: .";;-:~ .~,'.-".~-,,".'.:.'..:::.;".~,!( ~ -
1000
8. :.-- _, '.. 100
. > All:i!;;:il t. t;7 I I3
~ l!:i:: IN -
7".' .. ;~,'." "..
~,, ,,,,-
100 :.:-;:-I,,,I.::.;.., , , ,,,,,,, ;.:t, ,,, ,,,,,I , 14.5
10 ...... I(Xi 1000 10000 100000 1000000
Speed at bearing contact, mm/s
Figure 1.1 daol~deepS limitations for different types of lubricant
A1.1
1A Selection fo lubricant type
600
500
, o o ~ ~ ~ ~ -.THERMAL STABILITY LIMIT
.':-'::::::;:::'.~::.:-:i I ...... . ...... " ~ ~ ~ - ~ ~ (INSIGNIFICANT OXYGEN PRESENT) '
111 I-------
EFIL
NI THIS REGION DEPENDS NO
AMOUNT FO OXYGEN PRESENT AND ! .... ~ ~
~1 7J6u ~. 300 --'-----PRESENCE RO ABSENCE FO STSYLATAC --.+- , ~ , t i
lio
..-i ~,~~2~,~-yo,~:~?I . I!! I I ,I ~ , ~.o~~ ,~ .,o,,,o,xo ,, ~
u..,..o~o ~
m 200
E= ~~ w"~"~ ox.o~. ~o..~. ~, o.~,.,.~o
JIL
-.-i ~ ~ ~ ~ ~ ~ ~ ....... ~ ~ ~ ~ ~ .....
001 SLIO w..ou~ ..~.-o.,.o,,.~s: - ~ , I, : ~ ~ ........... ~ ~ ~ ~ :~ ~ ' - ~ ~ ~ ~ "' ~ ~~~:_.~~
671671~ ~'~'::'~'~ 'ii~~'ii~~'~'~' ~i!ii'~'~'~ ~iiiiiiiiii~ili~ii~!~ii~ii! N~I~_~ li~N~.....~:,::...:.......i!~
| I I I I I I i ! I 1 !
LOWER s?u.c~, TEMPERATURE vlsc?s,,,,, ,,,~,,,.~.,~ LIMIT IMPOSED ,,~v,.,oo,.,~,o.,,. YB POUR POINT, ~ WHICH VARIES WITH OIL-"
1 2 3 4 5 01 20 03 40 50 001 002 300 004 500 0001 2000 300040005000 | 10000
LIFE, h
Figure 2.1 Temperature limits for mineral oils
600 . . . . .
~_.~,_,.~__. 9(cid:12)LAMREHT-l
STABILITY LIMIT ROF POLYPHENYL ETHERS
50C __ ,.~,._..,.~._.,...,~..~ ,,
~ . . . . . . . . . . . . . 1 1 1 1 1 1 '
"~~" ~,-~\l\\', "" T ~ ~TAB~r'OXIDATION TIMIL ROF SREHTE~~.._~~LYNEHPYLOP ~,.....~ ' !I""
300
2~
. i,Jnu{ 200 ..... T ! "--_l LIMIT RCF ESTERS DNA SILICONES-
Z
1001,, I Ill i ~ "-- ~ - - " - - - ~'--" ~-,~,-,,-==
i
-100 LpOUR POINT LIMIT ROF SILICONES DNA ESTERS_
!
"1 2
3 4 5 10 20 30 40 50 001 200 300400500 0001 2000 300040005000 10000
LIFE, h
Figure 3.1 Temperature limits for some synthetic oils
A1.2
Selection fo lubricant type 1A
600
005
400 - ,
NOITADIXO LIMIT ROF SYNTHETIC GREASES PORD- POINT LIMIT ROF CITEHTNYS
HTIW UNLIMITED OXYGEN PRESENT GREASES WITH INORGANIC THICKENERS
o O 003
:;Xi 200 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ju ~ NOITADIXO FO MINERAL GREASES NI SIHT
:~ ~ ~ ~ REGION DEPENDS NO AMOUNT FO NEGYXO
UJ
-~ UPPER LIMIT IMPOSED YB DROP TNIOP ~ PRESENT
100-OF MINERAL LIO GREASES DEPENDS NO ITHICKENER ' -'-
/
WITH UNLIMITED OXYGEN
,
-100 ~ um,u=~^, P.o~^e=e ~ LOWEST LIMIT NO SYNTHETIC GREASES
IMPOSED T O R Q' YBU HEG' IH, I I ! DESOPMli I I YB IHIGH I EUQROT I I [
1 2 3 4 5 01 20 03 40 05 001 002 300 400500 0001 2000 300040005000 01 000
LIFE, h
Figure 4.1 Temperature limits for greases. nI many
cases eht grease life will be controlled by volatility or
The effective viscosity of a lubricant in a bearing may be
migration. This cannot be depicted simply, as it varies
different from the quoted viscosity measured by a
with pressure and eht degree of ventilation, but ni
standard test method, and the difference depends on the
general eht limits may be slightly below eht oxidation
shear rate in the bearing.
stimil
400 Typical SAE 30 mineral oil at 20~
~ ~ ( k ~ ~ ~ ~
Note: A steep slope shows a rapid j
change of viscosity with I
temperature and therefore I
10000 a reduced operating J OO3
temperature range, and si
~ ~ undesirable
,,, I . . . . . I
0001 ~ enOC~S _
%,,,
;~ 200 ~_.
8 raehS rate in raehS rate in
' 100 50 ~ ~ ' ~ , " ~ ' ~ ~ "~200 --- standard test ~ sgniraeb may --- -' -
methods si low eb high
o_ , .... , 4.J
o 20 , i
' i
I
2 ' X
f
. , o ...
xl 001
~ 5 . . . . .
4
I
Typical EAS 20/50 mineral oil at 100~
I I i i l i i I i | | | i i i i i i | I J J
30 20 10 0 10 20 30 40 50 60 70 80 90100120 1
Temperature, o C 100 1000 10000 100000 1000000
raehS rate (s -1 )
Figure 5.1 Viscosity/temperature characteristics of
various oils Figure 6.1 Variation of viscosity with shear rate
A1.3
A2 Mineral oils
CLASSIFICATION Traditional use
Mineral oils are basically hydrocarbons, but all contain Dating from before viscosity could be measured accu-
thousands of different types of varying structure, molec- rately, mineral oils were roughly classified into viscosity
ular weight and volatility, as well as minor but important grades by their typical uses as follows:
amounts of hydrocarbon derivatives containing one or
Spindle oils Low viscosity oils (e.g. below about
more of the elements nitrogen, oxygen and sulphur.
0.01Ns/m 2 at 60~ suitable for the
They are classified in various ways as follows.
lubrication of high-speed beatings
such as textile spindles.
Types of crude petroleum Light machine oils Medium viscosity oils (e.g. 0.01-0.02
Ns/m )2 at 60~ suitable for machin-
Paraffinic Contains significant amounts of waxy hydro-
ery running at moderate speeds.
carbons and has 'wax' pour point (see
Heavy machine oils Higher viscosity oils (e.g. 0.02-0.10
below) but little or no asphaltic matter.
Ns/m )2 at 60~ suitable for slow-
Their naphthenes have long side-chains.
moving machinery.
Naphthenic Contains asphaltic matter in least volatile
Cylinder oils Suitable for the lubrication of steam
fractions, but little or no wax. Their naph-
engine cylinder; viscosities from 0.12
thenes have short side-chains. Has 'viscosity'
to 0.3 Ns/m 2 at 60~
pour point.
Mixed base Contains both waxy and asphaltic materials.
Their naphthenes have moderate to long
sidechains. Has 'wax' pour point.
Hydrocarbon types
Viscosity index
The various hydrocarbon types are classified as follows:
Lubricating oils are also commonly classified by their
change in kinematic viscosity with temperature, i.e. by (a) Chemically saturated (i.e. no double valence bonds)
their kinematic viscosity index or KVI. Formerly, KVIs straight and branched chain. (Paraffins or alkanes.)
ranged between 0 and 100 only, the higher figures )b( Saturated 5- and 6-membered tings with attached
representing lower degrees of viscosity change with side-chains of various lengths up to 20 carbon atoms
temperature, but nowadays oils may be obtained with long. (Naphthenes.)
KVIs outside these limits. They are generally grouped )c( As (b) but also containing ,1 2 or more 6-membered
into high, medium and low, as in Table 2.1. unsaturated ring groups, i.e. containing double
valence bonds, e.g. mono-aromafics, di-aromatics,
polynuclear aromatics, respectively.
Table 2.1 Classification by viscosity index A typical paraffinic lubricating oil may have these
hydrocarbon types in the proportions given in Table 2.2.
puorG citameniK dscosity
xedni
Low viscosity index (LVI) Below 35 Table 2.2 Hydrocarbon types in Venezuelan 95 VI
solvent extracted and dewaxed distillate
Medium viscosity index (MVI) 35-80
High viscosity index (HVI) 80-110 nobracordyH t~s % emuloV
Very high viscosity index (VHVI) Over 011 Saturates f Paraffins 51
(KVI-- 105) ~ Naphthenes 60
I Mono-aromatics 81
Aromatics Di-aromatics 6
It should be noted, however, that in Table 2.5 viscosity Poly-aromatics 1
index has been determined from dynamic viscosities by
the method of Roelands, Blok and Vlugter, l since this is
a more fundamental system and allows truer comparison
between mineral oils. Except for low viscosity oils, when The VI of the saturates has a predominant influence on
DVIs are higher than KVIs, there is little difference the VI of the oil. In paraffinic oils the VI of the saturates
between IVK and DVI for mineral oils. may be 105-120 and 60-80 in naphthenic oils.
A2.1
Mineral oils A2
Structural group analyses
This is a useful way of accurately charactersing mineral atoms in aromatic groups (% CA), in naphthenic groups
oils and of obtaining a general picture of their structure (% CN), in paraffinic groups (% Cp), and the total
which is particularly relevant to physical properties, e.g. number (RT) of naphthenic and aromatic rings (RN and
increase of viscosity with pressure. From certain other )AR joined together. Table 2.3 presents examples on a
physical properties the statistical distribution of carbon number of typical oils.
Table 2.3 Typical structural group analyses (courtesy: Institution of Mechanical Engineers)
st,,c~
oamO % % %
Oil epyt at .5! ~6 ls'N .socsiV m2 ~" m naeMortar aC xC ,C AR NR TR
at 1 0O ~ thgiew
LVI spindle oil 0.926 0.0027 280 22 32 46 0.8 1.4 2.2
LVI heavy machine oil 0.943 0.0074 370 23 26 15 1.1 1.6 2.7
MVI light machine oil 0.882 0.0039 385 4 37 59 0.2 2.1 2.3
MVI heavy machine oil 0.910 0.0075 440 8 37 54 0.4 2.7 3.1
HVI light machine oil 0.871 0.0043 405 6 26 68 0.3 1.4 1.7
HVI heavy machine oil 0.883 0.0091 520 7 23 70 0.4 1.8 2.2
HVI cylinder oil 0.899 0.0268 685 8 22 70 0.7 2.3 3.0
Medicinal white oil 0.890 0.0065 445 0 42 58 0 2.8 2.8
REFINING used as such on some plain bearings subject to high
temperatures and as blending components in oils and
Distillation greases to form very viscous lubricants for open gears, etc.
Lubricants are produced from crude petroleum by
distillation according to the outline scheme given in Refining processes
Figure 2.1.
The distillates and residues are used to a minor extent as
The second distillation is carried out under vacuum to
such, but generally they are treated or refined both before
avoid subjecting the oil to temperatures over about 370~
and after vacuum distillation to fit them for the more
which would rapidly crack the oil.
stringent requirements. The principal processes listed in
The vacuum residues of naphthenic crudes are bitu-
Table 2.4 are selected to suit the type of crude oil and the
mens. These are not usually classified as lubricants but are
properties required.
Elimination of aromatics increases the VI of an oil. A
lightly refined naphthenic oil may be LVI but MVI if
l ceuo,: LIO, I
highly refined. Similarly a lightly refined mixed-base oil
may be MVI but HVI if highly refined. Elimination of
tC~TS~0 TA CIREHPSOMTA ~EssuREI
aromatics also reduces nitrogen, oxygen and sulphur
contents.
ETALLITSID "l ENILOSAG I The distillates and residues may be used alone or
__.
blended together. Additionally, minor amounts of fatty
,,
oils or of special oil-soluble chemicals (additives) are
I--' sAG, o"I
blended in to form additive engine oils, cutting oils, gear
:EUDISER DEPPOT EDURC oils, hydraulic oils, turbine oils, and so on, with superior
I
RO GNOL EUDISER I properties to plain oils, as discussed below. The tolerance
in blend viscosity for commercial branded oils is typically
LLITSID UNDER MUUCAV i +4% but official standards usually have wider limits, e.g.
~sPINDLE OIL!
+10% for ISO 3448.
l DISTILLATE >i THGIL! ENIHCAM L!O j
TROHSI IEUDISER I-~EAVY JLIO'ENIHCAM PHYSICAL PROPERTIES
EsABCINIFFARAP( fEDNILYCI~DEXIM)sLIoDNA R ISLIO
Viscos ity-te m perat u re
Figure 1.2 (courtesy: Institution of Mechanical Figure 2.4 illustrates the variation of viscosity with
Engineers) temperature for a series of oils with kinematic viscosity
A2.2
A2 Mineral oils
Table 2.4 Refining processes (Courtesy: Institution 5O000
of Mechanical Engineers)
ssecorP es~r~P 5000 '
De-waxing Removes waxy materials from paraf-
finic and mixed-base oils to prevent
early solidification when the oil is
cooled to low temperatures, i.e. to
reduce pour point OO5 k
A
De-asphalting Removes asphaltic matter, particu- E
larly from mixed-base short residues, 200,-
which would separate out at high Z E
and low temperatures and block v a. 100
oil-ways u
Solvent extraction Removes more highly aromatic mat- ~" 50
erials, chiefly the polyaromadcs, 0 ! VI
~~ -
in order to improve oxidation
stability
20
Hydrotreating Reduces sulphur content, and accord-
ing to severity, reduces aromatic IVK " ~
content by conversion to naphthenes 1 =
8.0
Acid treatment Now mainly used as additional to
other treatments to produce special 6.0
qualifies such as tramformer oils, ,5
white oils and medicinal oils 4.0
Earth treatment Mainly tO obtain rapid separation of 3.0 , , , , J
oil from water, i.e. good demulsi- -20 -10 0 10 20 60 80 100 120
bility Temperatu re, o C
Figure 2.2 150 grade ISO 3448 oils of 0 and 95 KVI
index of 95 (dynamic viscosity index 93). Figure 2.2 thenic oils, in contrast, simply become so viscous with
shows the difference between 150 Grade ISO 3448 oils decreasing temperature that they fail to flow, although
with KVIs of 0 and 95. no wax crystal structure develops. Paraffinic oils are
therefore said to have 'wax' pour points while naph-
thenic oils are said to have 'viscosity' pour points.
Viscosity-pressure
The viscosity of oils increases significantly under pres-
sure. Naphthenic oils are more affected than paraffinic ........................................... ................L .... i 4~
but, very roughly, both double their viscosity for every !iiiiiii!iiii!iii!ili!ii!iii!i!!iiiiiiiiliiii!iiii!iiiiiili!!i!ili iiiii!i!iiii!i!iiiiiii (cid:1)84
35MN/m 2 increase of pressure. Figure 2.3 gives an
impression of the variation in viscosity of an SAE 20 W
ISO 3448 or medium machine oil, HVI type, with both
temperature and pressure.
In elastohydrodynamic (ehl) formulae it is usually i!i!iiiiiiiiiiiiiiiiiIliiiiiiiiiiii, iiiiii!!iiiiiiiiiii !ii ililii!fi !
assumed that the viscosity increases exponentially with
pressure. Though in fact considerable deviations from an
exponential increase may occur at high pressures, the
,ooo
assumption is valid up to pressures which control ehl iii !iiii!iiiiiii!iii!iii
behaviour, i.e. about 35 MN/m .2 Typical pressure vis-
cosity coefficients are given in Table 2.5, together with
other physical properties.
Pour point
De-waxed paraffinic oils still contain 1% or so of waxy
hydrocarbons, whereas naphthenic oils only have traces
of them. At about 0~ according to the degree of
/ / .... o / 60oC (140,F)
dewaxing, the waxes in paraffinic oils crystallise out of
o'
solution and at about-10~ the crystals grow to the 20 000 tbf/in 10 000
extent that the remaining oil can no longer flow. This Figure 2.3 Variation of viscosity with temperature
temperature, or close to it, when determined under and pressure of an SAE 20 W (HVI) oil (Courtesy:
specified conditions is known as the pour point. Naph- Institution of Mechanical Engineers)
A2.3
04 i~ ~=~
04 C) l
> tr o r >: r Q. ~- I.~
3.5 4.0- 4.5- 5.0 6.01 7.0- 8.0- 9.0 10.0" 12.5 15 20- 25 30 40 50 75 100"- 150 200 300 400 500 750 1000 1500 2000 5000 7500 10000 15000 20000
U. A I r ! ~ I " '" ]- I- "= ...... ii~ __
I ur~
o
I
~ ~ ul r,, ~ -6~ -~>~ "--
~ i u. ~> _%'6 0 0r
~ ~e5 _ ,--'tel .~-~ m
~~ e'~ =5"
~ I
N
1"3
Temperature, 04 O Mineral
..._._.,,..._
A2.4 03 o (cid:12)9
~
~. o
oils
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