1. capítulo de livro - lubricant
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Chapter 3
LUBRICANTS
3.1 INTRODUCTION
Lubricants are put between two surfaces t o prevent direct contact. They may be
subdivided into solid lubricants and fluid lubricants.
Fluid lubricants are used in hydrostatic lubrication; these may be subdivided
into liquid lubricants and gaseous lubricants. Of the two, liquid lubricants are more
frequently employed, including water, also utilized in the first hydrostatic experi-
ments (ref.
1.1),
and liquid metals, especially sodium. But the liquid lubricants most
often employed are mineral lubricants. Nowadays synthetic lubricants are also
used.
Mineral lubricants are obtained from the distillation and refining processes of
crude petroleum, which is separated into fractions of progressively decreasing
volatility, with the elimination of the unwanted ones. Mineral oils are made up of
hydrocarbons, i.e., compounds
of
hydrogen and carbon.
Synthetic lubricants are produced by the substantial chemical modification of
raw materials, which may also be obtained from crude petroleum.
Mineral lubricants are mainly used in hydrostatic lubrication.
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36
HYDR OSTAT IC LUBRICATION
3.2 MINERAL LUBRICANTS
3.2.1 Types
Hydrocarbons, which mineral lubricants are mainly made up of, have three ba-
sic structures: paraffinic, naphthenic, and aromatic. Figure 3.1 shows their typical
configurations.
Paraffinic hydrocarbons generally predominate in mineral lubricants, followed
by naphthenic hydrocarbons. Aromatic hydrocarbons are usually few in number.
If the percentage of carbon present in paraffinic chains is considerably higher
than the percentage in naphthenic rings, the lubricant is called a paraffinic lubri-
cant; otherwise, it is called a naphthenic lubricant. Even a small amount of carbon
in aromatic rings helps boundary lubrication, owing
to
the presence
of
unsaturated
bonds.
- a -
- b -
-C
H
I
I
Fig.
3.1
Typical hydrocarbon configurations:a- paraffinic chain;b the so-called naphthenic ring;
c
aromatic ring.
3.2.2 Viscosity
Viscosity represents the internal friction of a fluid. Consider two layers in a
fluid, a distance dy apart (Fig. 3.2) . If we apply a tangential stress z along one of
these layers and observe a shear rate d u d y , with u as the velocity d x l d t , then we
may define the differential viscosity as p~ (ref. 3.1)
Note tha t Eqn 3.1 does not imply that the ratio 6zZx /&du dy ) s necessarily con-
stant throughout the fluid
or
during the time
of flow.
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LUBRICANTS
37
Fig.
3.2
Laminar shear between parallel planes in
a
fluid.
If
Sz,,/
& du ld y ) is constant and the shear rate is zero when the shear stress is
zero, then a flow is said to be
Newtonian.
The conditions for a Newtonian flow are:
3.2)
d u O when z = O
dY
A fluid which conforms to Eqn 3.2 is called Newtonian. Indeed, we owe Eqn 3.2
to Newton.
Figure
3.3
illustrates a number of ideal shear rate curves against the shear
stress of a Newtonian fluid the straight line through the origin),a pseudoplastic
fluid, a dilatant fluid, and a pseudoplastic material for example a grease) with a
yield
stress
ref.
3.1, 3.2).
I
Shear stress,Z
Fig.
3.3
Shear
rate -
shear s tress characteristics
of
materials:
A
- Newtonian fluid;
B -
pseudoplastic
fluid;C - dilatant fluid; D - pseudoplastic material.
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3 8 HYDROSTATIC LUBRICATION
Parameter
p,
defined by Eqn 3.2, is called dynamic viscosity. In system SI its
unit is Ndm2 or Pas, in system c.g.s. it is dynes/cm2o r poise.
Mineral lubricants and synthetic lubricants of low molecular weight are Newto-
nian in many practical working conditions.
In many fluid flow problems the ratio
3.3)
is used, where p is the density of the fluid,
v
is the kinematic viscosity, its SI unit is
m2/s and its c.g.s. unit is cm2/s o r Stoke (St).
In selecting a n oil for a given application, viscosity
is
a primary consideration,
especially from the point of view of its change with temperature. Various systems
are used to classify and identify oils according
t o
viscosity ranges, including the
Viscosity system for Industrial Fluid Lubricants , devised by IS0 (Std
3448)
nd
now coming into wide use. Viscosity systems establish a series of definite viscosity
levels as a common basis for specifying the viscosity of industrial fluid lubricants.
Reference viscosities are measured in mmVs
o r
cSt (centistokes) at the reference
temperature of 40C.The viscosity ranges and the corresponding marks
t o
classify
oils are shown in Table
3.1,
for
v=5.06+242
cSt.
For
comparison, the partial
S A E
(Society of Automotive Engineers) classification is also shown. The reference tem-
perature of the
SAE
classification is lOO C, nd
sUmx
W is intended for use in cases
where low ambient temperature is encountered.
TABLE
3.1
Viscosity System for Industrial Fluid Lubricants.
I
Viscosity System Grade Mid-Point viscosity Kinematic viscosity
Classification cSt (m 2 /s )a t
40C limits
and Identification cSt (mm%)at
40C
Min
Max
I S 0 VG 5
4.6 4.14 5.06
IS0 VG 7 6.8 6.12 7.48
ISOVG
10 10 9 .0 11.0
I S 0 VG 15 15 13.5 16.5
I S 0
VG 22 22 19.8 24.2
I S 0
VG
32 32 28.8 35.2
I S 0
VG 46 46 41 .4 50.6
I S 0 VG
68 68 61 .2 74.8
I S 0
VG
100
100 90.0
110
I S 0
VG
150 150 135 165
I S 0
VG 220 220 198 242
S A E
Classification
5w
low
2 0 w
30
5
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LUBRICANTS 39
(i) Viscos i ty- temperature.The viscosity of liquid lubricants decreases with in-
creasing temperature. Variations in temperature may be due t o external causes
and to energy dissipated because of viscous friction and changed to heat.
The following is an equation of the viscosity-temperature relationship, which is
simple but fairly accurate:
(3.4)
where po is the viscosity at reference temperature To and
p
is a constant determined
from measured values of the viscosity; its dimension is that of the inverse of tem-
perature.
Another widely-used equation is
log log(v a a
-
b logT
3.5)
where a and b are constants, and a varies with the viscosity level. For viscosities
over 1.5 cSt,
a
is
0.8;
above 1.5 cSt, a is 0.6. Using this type of log-log relationship,
charts have been worked out in which viscosity
is
represented by straight lines. In
Fig.
3.4
Eqn
3.5
is plotted for certain typical trade lubricants, which have been clas-
sified in conformity with IS0 (in actual fact IS0
VG 46
and
IS0 VG 68
fall a little
outside the kinematic viscosity limits at
40C).
The diagrams refer t o a
Viscos i t y
Index =lo0
o r a little higher (ref.
3.3).
Note that the log-log relationship compresses
the scale for high values of viscosity, so
a
graphic error of
1
may produce an error
of as much as
10
cSt.
Ever since the Thirties, the viscosity index ( V n has been of practical use for the
approximate estimation of the behavior of kinematic viscosity with temperature. It
makes i t possible to give a numerical value t o such behavior.
The viscosity index is based on two groups
of
oils. In one group, that is naph-
thenic in nature,
VZ=O
because of its sensitivity t o temperature; in the other, that is
paraffinic in nature,
VZ=100
because of its lower sensitivity.
Two oils are selected, one for each group, with the same viscosity at
100C
as the
oil to be tested. The viscosities of the three oils at
40C
are then evaluated. Taking
L
as the value of the oil with
VI=O,H
as that of the oil with
VI=lOO,
and
U
as that
of
the oil being tested, the viscosity index is given by the equation
L -u
vz=
---loo
L - H
3.6)
A t
present the
VZ
of mineral oils is often larger than
100,
and
as
Eqn
3.6
gives
largely inexact results for
VZ>lOO,
an empirical equation can be used:
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LUBRICANTS
41
-40 -20
0
20 40
60 80 100 120
140
160 180 200
TEMPERATURE .Dc
Fig. 3.5 Viscosity-temperaturebehavior for oils with different viscosity index
where
po is
the viscosity a t atmospheric pressure and is a constant determined
from measured values of viscosity; its dimension is that of the inverse
of
a pressure.
Indeed, pronounced deviations from the above relation are often encountered.
Naphthenic oils are more sensitive to pressure than paraffinic ones.
A t
hydrostatic pressures
viscosity may be considered t o be constant with
pressure.
3.2.3
Oiliness
Oiliness may be defined
as
the capacity of a fluid to adhere t o the surfaces of
materials. In usual conditions, especially if pressures are not high, the forces of
molecular adhesion are sufficient. If pressures increase, adsorption of the fluid on
the surfaces is then necessary. Adsorption
occurs
especially if polar molecules are
present in the fluid, i.e. molecules in which a permanent separation exists between
the positive and negative electric charges.
Mineral lubricants are not very oily, which is particularly unfavorable in
boundary lubrication.
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42 HYDROSTATIC LUBRICATION
3.2.4
Density
As is well known, the density of a liquid is the mass of a unit volume, generally
calculated at 15C. In paraffinic mineral oils p=0.85 0.89 kgfdm3;
in
naphthenie
mineral oils p=0.90-0.93 kg/dm3. Density varies with temperature and with
pressure.
(i) Thermal expansion. For a liquid, thermal expansion can be defined as the
property of being changed in density with temperature. It can be stated approxi-
mately by the equation
p T) po 11
-
a ( T -To)]
(3.8)
where a increases as p decreases; approximately: a=4.1.10-4
+
8.2.10-40C-1 or
p=0.22+0.01 Ns/m2.
In hydrostatic lubrication thermal expansion is often negligible.
(ii) Compressibi l i ty . The compressibility of a liquid can be defined a s the prop-
erty of being changed in density with pressure:
(3.9)
Compressibility can also be expressed as a change in volume with pressure; indeed,
if V is the volume (of a mass M f liquid, then from Eqn 3.9
1 d V
c=- - -
v
dP
(3.10)
Compressibility changes with pressure and temperature; it a lso changes with
molecular structure, but cannot be changed by means of additives, since it is a phys-
ical property of the base liquid.
Very often, instead of compressibility, its reciprocal
is
used: the
bulk modulus
KL. igure
3.6
(ref. 3.4) shows
a
method for predicting the bulk modulus of mineral
oils:
1) with density pT calculated a t ambient pressure (105 Pa) and a t the desired
temperature T, Fig. 5.6.a defines the bulk modulus at pressure 1380.105
Pa;
2
with this bulk modulus enter into Fig. 3.6.b: a vertical line at the intersec-
tion with the 1380.105 Pa line gives the modulus a t any other pressure
and at the selected temperature.
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LUBRlCANTS
43
- a - - b -
690
0 80 160 240 320 400 480 560
(-18 27 71 116 160 204 249 293)
TEMPERATUREFF, (OC)
Fig. 3.6 Bulk modulus of mineral oils
3450
3105
E
2760
cn
2415
2
2070
n
p 1725
1380
1035
/
/
/
p = 5
M N ~
For instance, let us consider a mineral oil with density p,=0.90 kg/dm3at ambi-
ent pressure and at temperature
T=40C:
ts
bulk modulus a t pressure -5 MPa and
at the same temperature is
K1=1680
MN/m2.
Alternatively, bulk modulus K Lcan be calculated by the semi-empirical equation
ref.
3.6)
K1= 1,44
+
0,15
logv)
[10.00235 20-n].1095.6p ,
Nm-2
3.11)
where v is the kinematic viscosity in cSt at a temperature of
20C
and at ambient
pressure;
T is
the temperature in
C;
p
is
the pressure in Pa.
So,
if the dynamic
viscosity of the oil in the previous example is
p 0 . 06
Ns/m2, from Eqn
3.11
the bulk
modulus is
K2=1570MN/m2.
Values of
K1
obtained from Fig
3.6
or by means of Eqn
3.11
are
fit
for high pres-
sures; nevertheless they can also be used, approximately, at mean and low pres-
sures the equation is preferable), as seen in the examples.
(iii)Gas solubility. Solubility of gases in liquids is
a
physical phenomenon which
can be evaluated by the ratio
3.12)
where Vgis the gas volume and Vi is the liquid volume, at the given partial pres-
sure of the gas and at the given temperature.
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4 4 HYDROSTATIC LUBRICATION
Solubility varies with pressure and temperature. In Fig. 3.7.a solubility of air
versus pressure is shown in a mineral oil (Mil-h-5606A) and in other liquids (ref.
3.5). Also
at
higher temperatures air solubility varies almost linearly. Figure 3.7.b
shows solubility versus temperature in the case of certain gases in
a
mineral oil
with p=850 kg/m3
The air dissolved m a y affect lubricant properties, such as viscosity which grows
worse. The air dissolved in an oil comes out of solution when temperature and pres-
sure decrease and may produce air bubbles and foam.
- a -
b-
MINERAL
OIL
0 2 4 6 8 1 0
0
25 50
75
100 125
PRESSURE, MP a TEMPERATURE PC
Fig. 3.7 Solubility
of
gase s versus: a- pressure; b- temperature.
(iv)Air entrainment. Common causes
of
entrained air in a liquid are, for ex-
ample, leaks in the pump suction
or
when the return line discharges liquid above
its surface level in the reservoir.
In any case, air is inevitably taken into a mineral oil as it passes into a lubricat-
ing system so that the oil in the reservoir may contain as much as 15%of dispersed
air (ref. 3.7). In a large and suitable reservoir this air should be given up and re-
duced, after a fairly long time, t o about 1.5%, and after a very long time t o about
0.5%. But the air bubble content is rarely reduced to the desired levels.
Air
bubbles, when compressed, go into solution, but not immediately. In Fig.
3.8, the percentage of air bubbles dissolved in a hydraulic oil is shown as a function
of time, for certain pressures (ref. 3.5). We see, for example, that for p=3 MPa, after
1
second, less than 10%of the air bubbles
go
into solution. Thus, as a result
of
the oil
velocity in the supply lines of hydrostatic systems (0.5t50 m/s and even more), these
percentages are generally
low.
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LUBRICANTS
45
40
0 1 2
3
4
5
T I M E
,
s
Fig 3.8
Rate of solution
of air
bubbles in a mineral oil.
Air viscosity is low, i.e. p=1.78.10-6 Ns/m2 a t l0C and 0,981.105 Pa; therefore,
entrained air affects the viscosity
of
oils. Fortunately, the effect
is
relatively slight,
and can be expressed by the empirical relation
lp
= (1+ 0.015 B )
where
B is
the percentage of bubble content,
p o
the viscosity of
oil
and pb the effective
viscosity of bubbly oil.
The
ir
bulk modulus
is
also low,
so
the entrained air affects the actual bulk
modulus
of
oils. The equation of state of a perfect gas (to which air may be assimi-
lated)
for
an adiabatic process
to
which the compression of air bubbles in mineral
oils may be compared) is
p VCP ~V =
const. (3.13)
where cp and
c v
are the specific heats a t constant pressure and constant volume,
respectively. If Eqn 5.13 s introduced into Eqn 5.10, the bulk modulus
Ka
of air
is
obtained
(3.14)
A t temperature T=40 C and at pressure p=0.981.105 Pa, we have cp=1.0048~103
J/kg C, and cv=0.717.103 J/kg C; thus we have cplcv=1.401,and Ka=1.37.105 Pa. In
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46
HYDROSTATIC LUBRICA
TlON
the context of hydrostatic lubrication, the variations of cp with temperature and
pressure are slight, while those of cv are negligible. Figure 3.9 shows cp/cvversus p
for certain values of T (ref. 3.8), and the bulk moduli of the air
at the
pressures in
Table
3.2
become those given in the same table.
0 1,96 3,92 5,aa 7,a5 9,ai
p , MNrn-
Fig. 3.9 Ratio of specific heats of air versus pressure, for certain temperature values.
TABLE 3.2
(v) Apparent bulk modulus. Air entrainment affects the properties of mineral
oils, especially bulk modulus, which greatly decreases. Indeed bulk moduli of
mineral oils are clearly much higher (even more than
lo
times) than those of air.
It
is
possible
t o
evaluate the apparent bulk modulus of a volume
Vi
of oil a s follows:
3.15)
in which
Va
is the volume of bubbly air uniformly entrained in oil, and whose bulk
modulus is
K
a t working pressure.
If
the lubricant contains
5 of
bubbly air at ambient pressure it must also be
taken into account that bubbles
of
other gases may also exist, which may be dis-
solved in oil in
a
greater proportion than that of air, as
is
shown in Fig.
3.71, or
Eqn
3.13
at pressures given in Table 3.2 that percentage is reduced to the values given in
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LUBRlCANTS
47
the same table. So, for Eqn 3.15, the apparent bulk moduli given in Table 3.2 may be
assigned to the oil a t those pressures.
The elastic deformation of the supply line may also influence the bulk modulus
of mineral oils. For a circular pipe, the internal pressure
ps
causes a change in
volume, which should be added to the change in volume due t o the compressibility of
the fluid when evaluating effective compressibility (Eqn 3.10) and hence the appar-
ent bulk modulus. The equivalent bulk modulus of a metallic pipe is, approxi-
mately,
(rs is the internal radius of the pipe, ra is the external one, E is the modulus of elas-
ticity and
v
is the Poisson ratio). Equation 3.15 may then be completed as follows:
(3.16)
For instance, let
us
consider a copper pipe, with ra=6 nun, rs=5 mm, and E=118
GPa, v=0.25; the equivalent bulk modulus of the pipe may be calculated as
Ks=10.2.109 Nlm2. For a steel pipe of the same dimensions, with E=206 GPa, v=0.3,
we have KS=17.6.1O9N/m2.
Generally, values of Ks for metallic pipes are much higher than the apparent
bulk modulus Kla of mineral oils (see Table
3.2) so
their influence may be disre-
garded. The same is not true
for
flexible pipes (also for high-pressure pipes made of
hard rubber o r FTFE with an interwoven sheet of stainless steel) as transpires from
existing experimental results. Figure 3.10.a (ref. 3.9) shows the considerable in-
crease in the inner volume of certain flexible hoses.
Obtaining realistic design values of the apparent bulk modulus of oil in hy-
draulic hoses is quite difficult. Values of Kla in the 70t350 MN/m2 range can be
found in the literature. Some results are shown in Fig. 3.10.b (ref. 3.5) or a woven
hose with rs=6.4 mm: the experimental data are clearly scattered. Recent design
practice in relation
t o
equipment dynamic noise reduction has tended to encourage
the use of hydraulic hoses in fluid power systems. This does not always seem con-
venient in hydrostatic systems, as a way of preventing possible dynamic instability.
Elastic deformations of instruments (manometers), pressure reservoirs
(accumulators) and other elements in the supply line may also influence
the
effec-
tive value of the apparent bulk modulus.
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48
48
nJ 4 02:
I
24
Y
16-
8 -
HYDROSTATIC LUBRICATlON
e
o o o
I
I
I I
- a -
10 15 20 0 4 8 12 16
20
PRESSURE,
MNrn-
PRESSURE,
MPa
Fig.
3.10
Flexible pipes: a- Inner volume variation (for unit length) versus pressure; (i) internal
radius
rs=5 mm,
rated pressure
pN=26
MPa; (ii)
rs=6.5
mm,
pN=26
MPa; (iii) rs=5 mm,
p ~ = l l
MPa. b- Apparent bulk modulus of lubricant versus pressure: rs=6.4 mm, SAE R2 Hose.
(vi)
Foaming.
The foaming of a liquid
is
due to the
air
bubbles tha t collect above
its surface. Common causes of foam are the same, but even greater, as in the case
of entrained
air.
Foam in a lubricating system can cause a decrease in pump efficiency, vibra-
tions, and above all inadequate lubrication.
(vii)
Cavitat ion.
In fluid systems gaseous cavitation refers to the formation in
the liquid of cavities that may contain air
or
other gases. Vaporous cavitation
refers
t o
the fact that, if pressure is reduced far enough, the liquid will vaporize and
will form vapor cavities (mineral oil vapors may contain volatile fractions
of
lubri-
cants). The vapor pressure of a liquid depends on
its
temperature and decreases
with
it.
A t
atmospheric pressure water boils a t 100C,
so
its vapor pressure is 1.0128
bar; at 21.1 C
its
vapor pressure is reduced to 0.025 bar. The vapor pressure of
mineral oils is much lower than that of water, typically 6.10-4 bar
at
4OOC; hence
cavitation is less likely to occur in the case of these liquids. In Fig.
3.11
the vapor
pressure of certain liquids
is
given as a function of temperature (ref.
3.5).
Cavities are well known to be associated with nucleation centers such as micro-
scopic gas particles
or
microscopic solid particles which gases join to), and their
development is caused by the rapid growth of these nuclei. Hydraulic liquids used in
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LUBRICANTS
49
i o r / 1
0
I
E
E
m
B
a
>
40 80 120 160 200 280 360
(494
26,7 49 71 93 138 1821
TEMPERATURE,
OF, OC 1
Fig.
3.11 Vapor pressure versus temperature..
conventional systems contain sufficient nuclei to ensure that cavitation will occur
when pressure is reduced to vapor pressure.
Cavitation damages hydraulic machinery and systems. Wear rate in particular
can be greatly accelerated if cavitation erosion develops. Cavitation may also in-
crease viscosity and reduce the bulk modulus of
oils.
In hydrostatic systems cavitation may also occur in the
sills
and in the recesses
where depression occurs, and in the recesses where turbulence occurs, which also
favours the formation of gases.
3.2.5 Thermal properties
(i)Specific heat.
Specific heat in mineral oils varies linearly with temperature;
it is:
-
for
naphthenic oils, c=1850-2120 J/kg C from 30
to
100OC;
-
for paraffinic oils, c=1880-2170 J/kgC from 30
to
100C.
(ii)
Thermal conductiuity. Thermal conductivity in mineral oils is: 0.133-0.123
Wm/m C from 30
o
100OC;
it
also varies linearly with temperature.
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5 0 HYDROSTATIC LUBRICATION
3 2 6 Other properties
Pour -po in t
is
the temperature at which an oil ceases to flow freely. This is
caused by the formation
of
crystals, mainly of a paraffinic type. The pour-point of
paraffinic oils is a t about -1O C, that of naphthenic oils a t about -40C.
Flash-point is the lowest temperature a t which the vapors given off by an oil
ignite momentarily on the application of a small flame. The flash-point of naph-
thenic oils is a t about 170 C,that of paraffinic oils at about 190C.
Acidi ty .
Low acidity is advantageous or reducing corrosion.
Oxida t ion . High stability to oxidation is advantageous, because one cause of
deterioration in lubricant oils is the formation of oxidation products. This also leads
t o
a reduction
of
the life of the lubricant and
to
corrosive effects.
Thermal decomposit ion. In the presence of oxygen, high temperatures may pro-
duce the thermal decomposition of mineral lubricants, which shortens their life.
Figure 3 .12 gives the approximate time-temperature characteristics of refined min-
eral lubricants, including oxidation (ref. 3.8).
10
10
Li fe ,
h
Fig 3.1 2 Approximate life-temperature characteristicsof a mineral oil: A - oil without anti-oxidant;
B - oil with anti-oxidant.
3.2.7
Additives
Nowadays lubricants often have chemical compounds added t o them to improve
them.
Viscosity index improvers
are generally organic polymers which are soluble in
oils, with a high molecular weight, such as polymethylmetacrilates. They cause a
decrease
or
a small increase in viscosity a t low temperatures, and a substantial
increase a t high temperatures. See also Fig. 3.5 and Fig. 3.13 (ref. 3.10).
Oiliness
improvers are, for example, fatty acids. They have polar molecules,
with a -CH3 group a t one end and a -C02H group at the other. This latter group
would be adsorbed on metal surfaces. Actually, owing to surface motion, and in the
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LUBRICANTS
5 1
10
1
.
In
0.003
0 50 100
120
I
.
In
0.003
0 50 100
120
Temperature,
Fig. 3.13. Viscosity-temperature characteristics o f A
-
a plain mineral oil;
B -
a mineral oil with a
viscosity index improver; C
-
a silicone fluid.
presence of a metal acting as a catalyst, oiliness improvers seem to change into
more complex compounds.
Foam additives
decompose and, therefore, reduce foam. Common foam decom-
posers include, for instance, silicones and polyacrylates, but the best way to reduce
foam is a suitable mechanical design.
Pour-point depressants
are generally complex polymers which coat the paraf-
finic crystals, thereby preventing them from increasing.
Oxidation inhibi tors, such
as
certain phenols, amines and olefines, prevent o r
reduce the formation
of
oxidation products. They also prolong the life
of
the lubri-
cant and act as corrosion inhibitors.
Corrosion inhibitors.
Rust, a hydrate iron oxide,
is a
widespread form of corro-
sion. Corrosion inhibitors, such as sulphonates, generally form
a
protective coating
on metal surfaces.
Many others additives, such as detergent , d i spers ant and extreme-pressure
additives, are used in lubrication; but they are of little importance in hydrostatic
lubrication.
More than one additive may be used a t the same time.
It
must be borne in mind,
however, that indiscriminate mixing can produce undesired interactions.
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5 2
HYDROSTATIC LUBRlCATlON
3.3
SYNTHETIC LUBRICANTS
The performance
of
synthetic lubricants is better than that of mineral lubri-
cants, but the former are much more expensive. They are often used in extreme
conditions, for instance in cases of high pressure or temperature.
Synthetic lubricants include the following:
Synthetic hydrocarbons;
the polyolefins and hydrobenzene in particular,
Organic esters;
those of dibasic acids in particular, which also have very good
Phosphatic esters,
whose oiliness is very good and whose thermal stability is
Polyglicols,
with very good oiliness, a high VZ and also fluidity a t low temper-
atures.
Silicones (with a polymer-like structure, in which the carbon is replaced by
silicon). They have a high VZ (see also Fig. 3.131, a high flash-point, a low pour-
point, high thermal stability and oxidation stability and a good anti-foam perfor-
mance. On the other hand, their oiliness is poor.
which have very good fluidity a t low temperatures, and a very good VZ.
fluidity a t low temperatures, and good thermal stability.
fair.
Various synthetic lubricants may be used
as
additives.
On lubricants see also ref. 3.11.
REFERENCES
3.1
39
3.3
3.4
3.6
3.6
3 7
Dorinson
A.,
Ludema
K.
C.;
Mechanics and Chem istry in Lubrication ;
Else-
vier, Amsterdam, 1985; 634 pp.
O'Connor
J.,
Boyd
J.; Standard Handbook of Lubrication Engineering;
Mc
Graw-Hill, New York, 1968.
Wills J. G.; Lubrication Fundamentals; M. Dekker Inc., New York, 1980; 465
PP.
Booser E.
R.; Handbook o Lubrication, 2nd Vol . ;
CRS
Press, Boca Raton
(Florida), 1984; 689 pp.
McCloy D., Martin H.
R.;
Control of Fluid Power; Ellis Horwood Ltd., Chich-
ester, 1980;
505
pp.
Liste des Caractkristiques Exigkes pour les Fluides Olkohydrau liques; CETOP
(ComitB Europben des Transmissions Olbohydrauliques et Pneumatiques),
London, 1971.
Fowle T;
Aeration in Lubricating Oils;
Tribology International,
14
(19811, 151-
157.
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LUBRICANTS 53
3.8 Raznyevich K ; ables et Diagrammes Thermodynamiques; Eyrolles, Didion,
1970.
3.9 Speich
H.,
Bucciarelli A.; LOteodinamica; Techniche nuove, 1971; 727 pp.
3.10 Neale
J.
M ;
ribology Handbook;
Butterworths, London, 1973.
3.11 Lansdown
A. R.;
Lubrication.
A
Practical Guide to Lubricant Selection;
Pergamon Press, Oxford, 1982; 252 pp.
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