1. capítulo de livro - lubricant

8/12/2019 1. Captulo de Livro - Lubricant

1/19

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.


2/19

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.


3/19

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.


4/19

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


5/19

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:


6/19


7/19

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.


8/19

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.


9/19

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.


10/19


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.


11/19

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


12/19

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


13/19

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.


14/19

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


15/19

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.


16/19


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


17/19

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.


18/19

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.


19/19

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.

1. capítulo de livro - lubricant

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