Lubricants

GBH Enterprises, Ltd.

Engineering Design Guide:
GBHE-EDG-MAC-5701

Lubricants

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Engineering Design Guide:

Lubricants

CONTENTS

SECTION

0

INTRODUCTION

1

SCOPE

1

2

LUBRICATION BASICS

2

2.1
2.2

Hydrostatic Fluid Film Lubrication

2.3

Hydrodynamic Fluid Film Lubrication

2.4

Boundary Lubrication

2.5

3

Basic Functions of a Lubricant

Mixed Lubrication

VISCOSITY

3

3.1

General

3.2

Dynamic Viscosity

3.3

Kinematic Viscosity

3.4

Measurement of Viscosity

3.5

Viscosity Classification of Lubricants

3.6

Viscosity Index

3.7

Viscosity Change with Pressure


4

MINERAL OILS

4

4.1
4.2

British Standard 4475 Commentary

4.3

Oil Additives

4.4

5

General Characteristics

Synthetic Oils

GREASES
5.1

Composition

5.2

5

Properties

6

SOLID LUBRICANTS

6

7

SELECTION OF LUBRICANTS

7

8

OPERATING FACTORS

8

8.1

Filtration

8.2

Operating Temperatures

8.3

Total Loss Lubrication Systems


9

LUBRICANT SUPPLY AND SCHEDULING
9.1

Selection of Supplier

9.2

9

Lubrication Schedules

10

HEATH AND SAFETY

10

11

MONITORING & MAINTENANCE OF OIL IN SERVICE

11

11.1

Analyze or Change?

11.2

Visual Analysis

1 I.3

Laboratory Analysis

11.4 Contamination Problems

BIBLIOGRAPHY

APPENDICES

A

VISCOSITY EQUIVALENTS

B

SYMBOLS AND PREFERRED UNITS


FIGURES

I

LUBRICANT CHANGE PERIODS AND TESTS

2

CHARACTERISTICS OF MINERAL LUBRICATING OILS
VG32 TO VG 460.

3

SERVICE MONITORING AND MAINTENANCE OF OIL IN
SERVICE ON LARGE SYSTEMS

TABLES

1

ISO VISCOSITY CLASSIFICATION

2

OILS TO BS 4475 RECOMMENDED FOR USE BY GBHE

3

SUGGESTED OIL CHANGE PERIODS FOR SMALL INDUSTRIAL
SYSTEMS

4

VISUAL EXAMINATION OF USED LUBRICATING OILS

5

SUMMARY OF ROUTINE ANALYTICAL TESTS FOR INDUSTRIAL OILS

DOCUMENTS REFERRED TO IN THIS ENGINEERING DESIGN GUIDE


0

INTRODUCTION

Tribology means the study of friction, wear, lubrication and bearing design. It has
been an internationally accepted title since 1966. The word is derived from the
Greek:
'tribos'

-

rubbing

'ology'

-

study of

Extensive research has given a considerable theoretical understanding of the
lubrication aspects of tribilogy; most problems arise through the failure to apply
this knowledge.
Observations in industry indicate that:
(a)

Few engineers have an adequate working knowledge of the subject.

(b)

There is a need to acquire experience to influence trends in design.

(c)

Engineering developments lead inevitably to more onerous operating
conditions.

1

SCOPE

Tribology is an interdisciplinary subject interacting surfaces in relative motion.
This Engineering Design Guide is concerned only with the study of lubricants.

2

LUBRICATION BASICS

2.1

Basic Functions of a Lubricant

These are:
(a)

Reduction or elimination of wear

(b)

Reduction of frictional forces

(c)

Dissipation of frictional heat


(d)

Prevention of corrosion of bearing surfaces.

(e)

Hydraulic fluid for power transmission.

2.2

Hydrostatic Fluid Film Lubrication

In this case the force between the contacting surfaces is balanced by pressure in
a separating film of fluid.
The pressure in the film is generated externally.

2.3

Hydrodynamic Fluid Film Lubrication

In hydrodynamic lubrication, the pressure in the film is generated by the relative
movement of the surfaces. This is usually a sliding motion with the surface
having a slight convergent wedge in the direction of motion, but it can also be
through the surfaces approaching normally. This latter form, known as 'squeezefilm lubrication', enables high, short duration loads to be sustained without film
breakdown and is of great importance in reciprocating machines.
For non-conformed contacts, sufficient local elastic deformation may occur for
fluid film separation to be maintained at very small surface separation. This is
known as “elastohydrodynamic lubrication” (EHL). With materials having a high
elastic modulus sufficient pressure is generated between the surfaces to give a
marked increase in the lubricant viscosity that helps to maintain a separating film;
this occurs in rolling bearings, gears and cams. EHL also occurs with materials of
low elastic modulus without viscosity increase; this mechanism occurs in rubber
seals, human joints and is also responsible for the aquaplaning of tires.

2.4

Boundary Lubrication

A thin solid or semi-solid film separates the contacting asperities on the surfaces.


2.5

Mixed Lubrication

An intermediate condition between hydrodynamic and boundary lubrication
where part of the load is supported by boundary lubricated solid contacts and
part by hydrodynamic films.
Optimum conditions of lubrication exist when the sliding surfaces are completely
separated. At low speeds only boundary lubrication will occur and this is provided
either by a lubricating solid or an adsorbed film of liquid lubricant. Even in
mechanisms designed for full hydrodynamic lubrication, boundary lubrication
will occur at starting and stopping or when the direction of motion is reversed.
Under boundary conditions some wear of the surfaces is inevitable and in the
case of heavy rotors hydrostatic jacking oil may be applied to the bearings at
starting to replace the friction and the possibility of damage until there is sufficient
velocity to generate a full hydrodynamic film.

3

VISCOSITY

3.1

General

Viscosity is the most important single property of lubricating oil. Fundamentally it
is resistance of the fluid to shear. It is dependent on temperature and, to a lesser
extent, pressure and is usually measured at 40oC (previously 37.8oC or 100oF)
and atmospheric pressure. This value is known as the Viscosity Grade.
In lubrication duties the flow of oil is usually laminar. Transition from viscous to
turbulent flow occurs as the velocity is increased above a critical value; this only
occurs with thick lubricant films and high surface velocities, e.g. in large power
generating steam turbines with bearing diameters above 500 mm.

3.2

Dynamic Viscosity

For most lubricants the shear stress is proportional to the rate of shear, the
constant 1 being known as the dynamic viscosity

ᶯ=

shear stress
Rate of shear


The units of

ᶯ are:

(a) CGS System:

dyn s =
m2

Poise (P)

In practice, the centipoise cP (i.e. 0.01 P) is used.

Ns = 103 cP
m2

(b)

SI System:

3.3

Kinematic Viscosity

Kinematic Viscosity is defined as:

Ѵ = Dynamic viscosity

=

Density

ᶯ
ρ

The units of Ѵ are:
(a) CGS System:

cm2 = Stokes (St)
s
The practical unit is the centistoke (cSt) which equals 0.01 St. Note that
cSt = mm2/s.

(b)

SI System:

m2 = 106 cSt
s


3.4

Measurement of Viscosity

The suspended level Capillary Viscometer is a commonly used measuring
device. It gives results directly in kinematic viscosity and is based on the time it
takes for a definite quantity of liquid to flow by gravity through a capillary tube.
Efflux Viscometers use an orifice in place of the capillary tube, the chief forms
being:
(a)

Saybolt Viscometer
This is used in the USA. There are two standard sizes, Universal and
Furol. The viscosity is the time in seconds for efflux, e.g. Saybolt Universal
Seconds, SSU. Normal reference temperatures are 100°F and 210°F.

(b)

Redwood Viscometer
This is used in the UK. There are two sizes, Redwood No 1 and Redwood
No 2. The viscosity is the time in seconds for efflux, e.g. Redwood 1
seconds. Normal reference temperatures are 90°F, 140°F, 200°F.

(c)

Engler Viscometer
This is used in Europe. The viscosity is the ratio of the time of efflux of the
oil to that of water at this same condition. Viscosity is quoted as degrees
Engler, e.g. °E. Normal reference temperature is 50°C.

These empirical units are slowly going out of use.
An equivalent table is given in Appendix A.

3.5

Viscosity Classification of Lubricants

A system for this classification of industrial liquid lubricants was agreed in 1975
(ISO 3448, BS 4231). This defines 18 Viscosity Grades in the range 2 to 1500
cSt at 40°C; each grade is designated by the nearest whole number to its midpoint viscosity in centistokes and a range of + 10 of the midpoint value is
permitted. The viscosity of each grade is approximately 50% greater than that of
the preceding grade. Table I shows this classification.


TABLE 1

ISO VISCOSITY CLASSIFICATION

A quite separate classification is used for engine oils (Society of Automotive
Engineers, SAE J300). This uses 100oC as a reference temperature and also
gives limits at low temperatures where mineral oils show non-Newtonian
viscosities (this is to cover cold start conditions). Arbitrary Grade Numbers are
used with the suffix W for the low temperature limits. A normal mineral oil will
have the same Grade Number as both temperature limits (e.g. 20/20W) but by
the incorporation of certain additives it is possible to produce oils with different

low and high temperature grade numbers (e.g. 10W/40). Such oils are called
multigrade oils.
SAE Viscosity Grades are sometimes used in recommendations for industrial
machines. Because of the different method of classification ISO Viscosity Grades
and SAE Viscosity Grades are not strictly comparable; however for practical
purposes the following relationships apply:
SAE GRADE
10
20
30
40
50
60*
70*

ISO GRADE

Transmission

32
68
100
150
220
320
460

SAE 75
SAE 80
SAE 90
SAE 140

32
150
460
1000

* These Grades are obsolescent.

3.6

Viscosity Index

The Viscosity Index (VI) concept was devised in 1929 as a way of expressing the
viscosity/temperature relationship, for a particular oil, by a single empirical
number. Reference oils were chosen having, what were considered at the time,
maximum and minimum limits of viscosity/temperature sensitivity and they were
assigned the end points of an 0 to 100 scale of VI.
The VI of oil is calculated from determined viscosities at 40°C and 100oC and the
use of tables which give the viscosities at 40oC of the 0, VI and 100 VI reference
oils which have the same viscosity at 100oC as the oil under test.
Oils with a low VI have a large change of viscosity with temperature.
Oils with a high VI have a small change of viscosity with temperature.


Many oils are now produced with characteristics outside the upper and lower
limits of the above scale. For example all "winter" oils have a VI of about 120. A
new method has been developed to deal with oils over 100 VI (Viscosity Index
Extension VIE) which gives similar values at 100 but greater differentiation at
larger VI levels. Other more suitable methods, such as the Viscosity
Temperature Coefficient (VTC) has been proposed but are rarely met
in practice.
A law suggested by Walther gives a theoretical relationship and is used in the
ASTM chart.
log E log (Ѵ + 0.6) = a - b log T
where Ѵ
T
a+b

is the oil viscosity centistokes
is the temperature (absolute)
are constants for the particular oil.

For straight mineral oils Fig. 2 gives a guide to this variation.

3.7

Viscosity Change with Pressure

Viscosity increases with pressure and this important in the elastohydrodynamic
regime of lubrication and in hydraulics. Viscosity change follows a quasiexponential law. For a typical mineral oil at 40oC, its viscosity will have doubled
by 350 bar and will have increased twenty fold by 1500 bar. Oils which show a
large change of viscosity with temperature (Low VI) show a large change with
pressure.
A theoretical relation is given below:

ᶯ = ᶯo eαP
This equation seriously over-estimates the viscosity above 3000 bar.
According to Wooster
ᶯ
α = (4.7 + 7.5 log to 10 0) 10-4


Where ᶯo is the viscosity at 1 bar in centipoise
P is the pressure in bar

4

MINERAL OILS

4.1

General Characteristics

Mineral oils are not pure chemicals, but consist of a very wide range of
hydrocarbon of varying molecular weight and type. The three main types are
paraffins, naphthenes and aromatics. The products of refining are a small range
of base stocks of different viscosities, which are then blended to produce oils of
the required viscosity.
In addition to carbon and hydrogen, mineral oils contain small amounts of sulfur,
usually less than 1%. This sulfur is not chemically active and normally of no
consequence as far as lubrication goes, however if it gets into process streams it
may lead to poisoning of catalysts etc.
A summary of most of the important properties of mineral oil lubricants is given
below:
(a)

Viscosity
Mineral lubricating oils can be manufactured in a continuous spectrum of
viscosity (viz 2 to 1500 cSt at 400C). In practice only a restricted
standardized range is produced.

(b)

Oxidation Stability
Oxidation (the reaction between hydrocarbons in the oil and oxygen in the
air) is a function of time and temperature. It is negligible with mineral oils
below 400C. Straight mineral oils can normally be used up to 600C, for
temperatures above this or where prolonged service is required oxidation
stability can be increased by the incorporation of anti-oxidant additives.
Oxidation is aggravated by the presence of water, metallic wear particles
and other contaminants and also by churning and agitation. Fig 1 gives an
indication of oil life as a function of temperature and access of air.


Oxidation is undesirable as it gives rise to:
(1)

Soluble products that increase the viscosity and may form deposits on
high temperature surfaces.

(2)

Insoluble products which can block oil holes, filters etc.

(c)

Acidity
Oils are neutral, but become acidic as a result of oxidation. The oxidation
acids are not corrosive but provide a measure of deterioration.

(d)

Demulsification
The ability of the oil to deal with water ingress (i.e. to separate out the
water), is controlled in new oil to an acceptable level.

(e)

Pour Point
Oils, when cooled sufficiently, form plastic waxy solids. Pour point is the
temperature at which an oil just flows and this imposes a lower design limit
on working temperature.
With the normal range of straight mineral oils, the pour point varies
between +6°C and -18°C. Oils with values down to -60°C are obtainable
and this is significant for the operation of refrigeration compressors.

(f)

Density
The density of most oils lies between 850 and 960 kg/m3 (at 15°C).
An approximation of the thermal expansion is given below:

ρ t = ρ0 ( 1 – βΔt )
where

ρ0 is the density at 150C
t = is the temperature rise °C
β = 9.9 - 1.8 log10

Kg/L

Kg/l

ᶯ = dynamic viscosity at 15°C in centipoise

The bulk modules of oil can be expressed as:
c = (72.5 - 9.67 log10.) 10-6
Where
c =

ᶯ
(g)

1 x
ρ

dρ or 1 x dV
dP
V dP

in bar -1

= oil viscosity in centipoise at I bar.

Specific Heat
The specific heat of mineral oils between 00 and 200 0C within
the specific gravity range 0.75 - 0.96 is obtained from:

s = 1.8 + 0.00366 t

KJ/kg 0C

add 2% for paraffinic oils
subtract 2% for naphthenic oils.
(h)

Thermal Conductivity
The thermal conductivity of mineral oils between -200 and
4000C within the specific gravity range 0.75 - 0.96 can be obtained from
the following formula:

K = 0.13 - 7t x 10-3

W/m0C

accuracy + 10%
(j)

Thermal Decomposition
Oils will break down above 3300C, even in the absence of oxygen, forming
carbon deposits and with a decrease in viscosity and flash point. (See Fig.
1).


(k)

Load Carrying
Boundary lubrication properties of straight mineral oils are adequate for
most industrial applications, including spur and helical gears. There are
exceptions, such as hypoid gears and high pressure ( > 100 bar) slidingvane hydraulic pumps where additives are essential.

(1)

Corrosion Protection
The small amounts of oxidation products in normal mineral oils give
reasonable protection for ferrous alloys against corrosion; oils containing
anti-oxidants require the incorporation of rust preventing additives.

(m)

Aeration
Some mechanisms easily permit air entrainment in the bulk of the oil or
foaming on the oil surface. Aeration is an undesirable situation which can
be eased by system design and suitable selection of oil. Certain additives
can be used to increase the rate of foam breakdown but they should be
applied with caution as they tend to reduce the rate of air release.
The viscosity of a gas oil mixture is higher than that for the oil along by the
factor (1 + 0.015 ) where is the volumetric percentage of the gas content.
The solubility of air in oil is sufficiently well given by

S = 1 ( 11 – 2 log10 Ѵ)
Where

Ѵ is the viscosity in centistokes
S is the volumetric percentage of air
P is the absolute pressure in bar
This formula holds over a wide temperature range.


(m)

Flash Point
This is the lowest temperature at which the vapor above an oil ignites
when exposed to a flame, using a standard method for testing. Lubricating
oils usually have flash points (closed) between 130°C and 250°C
depending on viscosity. Flash Point is normally only of significance in
lubricant manufacture as a control against the incorporation of low
viscosity, volatile components.

4.2

British Standard 4475 Commentary
British Standard 4475, 'Specification for Straight Mineral Lubricating Oils'
defines three quality levels of oil.
Table 2 gives a restricted selection of oils to BS 4475 and meets GBHE's
requirements for general purpose lubrication.

TABLE 2

OILS TO BS 4475 RECOMMENDED FOR USE BY GBHE

Typical temperature-viscosity characteristics are given in Fig 2.


4.3

Oil Additives

It is possible to modify the characteristics of a straight mineral oil by the
incorporation of small amounts of other materials (usually < 1%). Additive oils
should be used with discretion as the additives tend to be more reactive than the
base oil and can react with system components or contaminants giving rise to
harmful deposits. Additive oils are more expensive than straight oils.
Important types of additive are as follows:
(a)

Anti-oxidant
These reduce the rate of degradation and are advantageous in the
temperature range 60°C to 80°C. (See Fig 1). A side effect is that they
create a need for a corrosion inhibitor.

(b)

Extreme Pressure (EP)
Prevents welding and scuffing of sliding surfaces under severe operating
conditions. Principally used in gear lubricants; particularly when
associated with hypoid design and units subject to shock loading, and high
pressure hydraulic pumps. Some EP additives react with yellow metals
at ambient temperature and should be used only with steel systems. On
gears other than hypoid there is no evidence of any advantage of EP oils
over straight mineral oils.

(c)

Detergent/Dispersant
Used in engine oils to prevent carbon deposits. Have no application in
industrial machines other than engines.

The main application of additive oils is in large circulation systems (e.g. turboalternators and rotary compressors) where the use of an oil containing an antioxidant and rust inhibitor will increase the period between oil changes. In small
systems without filters the need for oil change is determined by contamination
rather than oxidation and straight oils provide the most economic choice. BS 489
covers a range of oxidation and rust-inhibited oils.
With straight mineral oils there is no problem with equivalents or mixing products
from different manufacturers. This is not the case with additive oils. It is thus
necessary to study the description of the oil and its application before selecting
an equivalent. Information on the composition of branded oils is not easy to
obtain. Makers recommendations and guarantees should not be overlooked.

Experience within the company shows that in all but a very small number of
applications, industrial machines can be satisfactorily lubricated by straight
mineral oil.
The lubrication requirements of reciprocating refrigerant compressors are met by
straight mineral oils, but they have special requirements to ensure absence of
solidification with inorganic refrigerants and adequate miscibility with halogenated
refrigerants. Suitable materials are covered by BS 2626 which covers five
viscosity grades.

4.4

Synthetic Oils

These are only occasionally used in industry. They are of value in extending the
temperature range of liquid film lubrication. In aircraft gas turbines, synthetic oils
have now completely displaced mineral oils. They are more expensive in all
cases.
The chief types are:
(a)

Organic Acid Esters
High VI, but only available in low viscosity grades, ( < 10 cSt at 400C). Can
be used up to 240 0C.

(b)

Phosphate Esters
Can be used up to 120 0C. High resistance to combustion. Strips paint
coatings. They have poor lubrication properties, particularly in high contact
load applications.

(c)

Polyglycols
High VI. Can be used up to 200 0C. Degradation products volatile.
Outstandingly good boundary lubricants and for this reason very effective
in worm gearboxes, allowing increased power transmission for the same
temperature use.


(d)

Synthetic Hydrocarbons (SHC)
Similar properties to mineral oil but sulfur-free and more resistant to
oxidation. Built up from selected pure chemical compounds and therefore
are free from "light" and "heavy" ends. This means the pour point is lower
and the flash point higher than those of a straight mineral oil of equal
nominal viscosity.

(e)

Silicones
Extremely high VI. Poor boundary lubricating properties, but have
extremely wide temperature range of application (-60 0C to 250 0C).

(f)

Fluorocarbons
Low VI. Can be used up to 150 0C. Extremely good thermal and chemical
stability; fire resistant and the only lubricant that can be used with liquid
oxygen. Very expensive and have poor lubrication and corrosion
preventive properties.

5.1

Composition

Most lubricating greases are based on mineral oils of Viscosity Grade 68 to 220
mixed with a thickener to give a semi-solid product. Metallic soaps are the most
common thickeners, but more recent developments include polyurea and
Bentonite clay. The mechanical stability of a grease is a function of the base oil
viscosity and the amount and type of thickener.
Important types of grease, in ascending order of cost, are listed below:
(a)

Calcium (Lime)
Excellent water resistance and corrosion preventive properties within the
temperature range. Maximum operating temperature 60°C (long term)
80°C (short term). Drop point 100°C (i.e. melt point under set conditions).


(b)

Lithium
Good water resistance and incorporates additives to give corrosion
preventive properties. Maximum operating temperature 80°C (long term),
l40°C (short term). Drop point l80°C. Standard grease for rolling contact
bearings.

(c)

Polyurea
Similar to lithium greases, but with better temperature resistance, 100°C
(long term); used particularly in the bearings of electric motors with Class
F insulation.

(d)

Bentonite Clay
Good water resistance and corrosion preventive properties. Maximum
operating temperature 200°C (short/long term). High temperature duties.
Poor lubricating properties and only suitable for rolling bearings at low
speeds.

(e)

Perfalkyl type. Very expensive, care should be taken in their use.

5.2

Properties

The main advantage of grease over oil is that it does not flow under its own
weight and hence allows simpler sealing arrangements. This is a major attraction
in rolling bearings, lubricated flexible couplings and small sealed-for-life units. On
the other hand because of this property grease does not circulate and cannot be
used like oil to remove frictional heat.
The most important physical property of grease is its consistency. This is
determined by a penetrometer: a weighted metal core is allowed to sink into a
sample of the grease at 24°C for a specific time and the distance it sinks,
measured in tenths of a millimeter, is termed the penetration. Greases are
classified in terms of their consistency using an arbitrary set of numbers
originating from the National Grease Resource Institute of America (NLGI) •


The relationship between NLGI and penetration is as follows:

NLGI Class

1

2

3

Penetration

310 265 220
-340 -295 -250

4
175
-205

5

6

130 85
-160 -115

In addition a range of partially thickened oils that flows in on their own weight are
also described as greases; in order of increasing consistency they are given the
NLGI Nos 00 and 0.
The semi-fluid greases are used in gearboxes, Classes 1 and 2 greases in
pumped systems, Classes 2 and 3 for rolling bearings with Class 3 grease
preferred for vertical applications. Other properties of interest are:
(a)

Shear Stability
The shear stability is determined by the change in penetration as received
and after working in a standard test apparatus. The worked penetration
should be in the same NLGI class as the unworked values.

(b)

Pumpability
Greases are non-Newtonian materials that is their viscosity varies with the
rate of shear. The viscosity at any particular rate of shear is known as the
Apparent Viscosity. Apparent Viscosities are required for estimating
pressure drops in pipe runs. Plots of Apparent Viscosity against rate
of shear can be obtained from the grease supplier.

(c)

Extreme Pressure Properties
EP additives can be incorporated into greases, but with some loss in
stability. EP greases are used in heavily loaded taper or spherical roller
bearings to prevent scuffing.


6

SOLID LUBRICANTS

These are used essentially for boundary lubrication and are no substitute for
mineral oils. They provide lubrication when temperatures are too high for mineral
oils, in situations where normal lubricants would be squeezed out, e.g. in
assembly compounds, thread release agents. Sometimes used as an additive to
conventional oils and greases.
Graphite and Molybdenum Disulphide are the most well known.

7

SELECTION OF LUBRICANTS

Machine manufacturers are conservative in their lubricant recommendations,
frequently specifying precise Viscosity Grades and the use of additive-type oils
when these are unnecessary. Most mechanisms are satisfactorily lubricated by
straight mineral oils. This allows considerable reduction in lubricant stocks that
not only reduces cost but reduces confusion and the risk of error.
Table 5 gives a guide to lubricants for normal industrial machines based on
experience in GBHE. Difficulties can arise during the warranty period of a
machine if the lubricant recommended by the manufacturer is not used. The
simplest way to overcome this is to specify a lubricant to BS 4475 at the
tendering stage and place the onus on the Vendor to satisfy the Purchaser of
cases in which this will not be acceptable.

8

OPERATING FACTORS

8.1

Filtration

With a new machine, system flushing is essential, using a separate charge to
remove the bulk of both liquid and solid contaminants and the temporary
corrosive preventive with which parts are coated prior to supply.


8.2

Operating Temperatures

The following checklist gives various temperatures associated with systems and
bearings:
(a)

Oil Reservoir
60°C (optimum), 70°C (maximum). Values set to prevent degradation of
mineral oils.

(b)

Oil Inlet to Bearing - Circulating System.
40°C to 50°C (typical range).
Where bacterial contamination may cause problems 45°C to 55°C should
be specified.

(c)

Oil Outlet from Bearing - Circulating System
60°C to 70°C (typical), 80°C (maximum). Values set to prevent
degradation of mineral oils.

(d)

Plain White Metal Bearing
Temperatures measured by thermocouple embedded in the white metal
with 1 mm of the bearing surface.
80°C (optimum), 100°C (normal maximum). Values set by the possibility of
chemical reaction between bearing white metal and the lubricant, or
contaminants in the lubricant, producing deposits and leading to failure.
130°C (accepted maximum for high speed bearings). Value set by the
limit of physical properties of white metal, the metal starts to soften.
Oil degradation is little influenced by short term high temperature peaks in
the bearing itself.

(e)

Rolling Bearings
120°C (maximum for standard production bearings). Value set by bearing
materials and lowering of oil viscosity.


Specially heat treated rolling bearings with some loss in load and carrying
capacity are available to operate at temperatures up to 230°C but need
synthetic oil.

8.3

Total Loss Lubrication Systems

It is often preferable, e.g. in secheurs, conveyors, mills etc with a large number of
widely separated moving parts to install a system for dispensing lubricant from a
central position, either grease or oil, and either manually or automatically in a
total loss system. Measuring valves are operated together or progressively, the
pump either being in continuous or intermittent service. Should a blockage occur
at anyone of the bearing points, it can be arranged that the resulting high
pressure can initiate a visual or audible alarm. Pumpability is important in a
centralized greasing system. Residence time of the grease or oil in the pipelines
can be considerable and lead to problems.

9

LUBRICANT SUPPLY AND SCHEDULING

9.1

Selection of Supplier

GBHE policy is to use straight mineral oils to BS4475 wherever possible;
experience shows that this normally amounts to about 60-70% of the requirement
on any particular plant or works. Contracts for oils to BS4475 should be awarded
to suppliers on the basis of competitive tendering.
The use of branded proprietary oils and greases and oils to particular
specifications obtained from the major manufacturers.
9.2

Lubrication Schedules

Lubricant schedules for new plants are drawn up by the Contractor or Machines
Manufacturers to obtain the maximum rationalization.
See Appendix B.


10

HEALTH AND SAFETY

Oil can affect health by being swallowed or inhaled as droplets or vapor and by
contact with skin or eyes. Prolonged contact with the skin can lead to dermatitis
or, more rarely, skin cancer. Providing straightforward precautions are observed,
there is not considered to be any risk.
As a routine, hands and forearms should be washed before commencing a work
period, barrier cream applied, and washed again at the end of the period. Cuts
and scrapes should be attended to without delay. Hands should never be
washed in petrol or similar solvent. Oil soaked rags should not be left in overall
pockets.
Data on Oral LD50 and Threshold Limit Values (TLV) are available from the oil
suppliers covering their particular products. The following leaflets are also
available:
SHW
SHW
SHW
SHW

11

295
295A
367
397

Effect on the Skin of Mineral Oil
Cancer of the Skin Caused by Oil
Dermatitis. A Cautionary Notice
Cautionary Notes: Effects of Mineral
Oil on Skin

MONITORING & MAINTENANCE OF OIL IN SERVICE

Lubricating oils deteriorate in service either because the lubricants degrade or
they become contaminated. The majority of the lubricant tests covered by IP,
ASTM, DIN etc., are aimed primarily at ensuring that the lubricant meets some
specification level rather than monitoring the condition of the oil in service
and are therefore not necessarily appropriate to the latter function.
A monitoring scheme may be used not only to assess the fitness of the oil for
further service but may indicate actions that can be taken to restore the condition
of the oil.


11.1

Analyze or Change?

Before adopting any monitoring system the cost of monitoring has to be
considered. In the case of small systems it may be more economic to resort to
changing the oil at some routine rather than attempt to assess its fitness for
further service. The distinction between small and large systems is not precise
but 100 - 200 liters may be taken as a guide. In fact few systems fall into this
size: self-contained systems are usually less than 50 liters, circulation
systems more than 1000 liters.
Change periods for small systems have to be determined by practical
experience. Suitable periods for typical industrial equipment are indicated in
Table 3.

TABLE 3 - SUGGESTED OIL CHANGE PERIODS FOR SMALL INDUSTRIAL
SYSTEMS

For the larger systems a 2-tier system of monitoring is suggested consisting of a
visual examination at say weekly intervals, backed up by oil analysis at some
longer routine or as indicated necessary by the visual analysis. Fig 3.


11.2

Visual Analysis

For visual analysis a sample of the circulating oil should be taken in a clean glass
bottle (2-4 oz). It is helpful to retain a sample of the new oil and the previous
week's sample so that a direct comparison can be made to see if any change has
occurred, the samples being kept in a cupboard out of direct sunlight. If the
oil is clean and has not noticeably darkened no action needs to be taken. If it is
cloudy or opaque it should be stood for 30 – 60 minutes, preferably on a radiator
where it can be heated to about 600C and re-examined. Table 4 indicates a
scheme for visual analysis and the action to be taken.
Although primarily of use in large systems, visual examination can also be of use
of course in examining the oil from small systems where it is suspected that
something may have happened to the oil before the routine change period has
been reached.

11.3

Laboratory Analysis

Laboratory analysis is expensive but it is justified with large systems. In the case
of Works without their own laboratory facilities the oil supplier may be prepared to
help or it may be necessary to use an external laboratory.
The following indicates the most useful tests for oils in service.
A 6-monthly routine is normally sufficient.
(a)

Viscosity
Most systems can tolerate a wide variation in viscosity.
Nevertheless a routine test should be carried out to check that the wrong
oil has not been used for topping up and there has not been contamination
with oil-soluble material. The viscosity at 40'C should not differ more than
+ 15% from the new oil value. (e.g. ISO VG 68. 57.8 to 78.2 cSt)


TABLE 4 VISUAL EXAMINATIONS OF USED LUBRICATING OILS

NOTES:
1

This includes paper and felt filters, not wire mesh strainers.

2

Both foams (mixtures of oil and air) and emulsions (mixtures of oil and
water) render the oil opaque. When an opaque sample is received it
should be stood for 1 hour, preferably at 60°C (an office radiator provides
a convenient source of heat): a foam breaks down, liberating the gas and
leaving a clear oil; a stable emulsion persists after this time; a less stable
emulsion shows a separated layer of water below the oil.

3

In a dark oil solids can be seen by inverting the bottle and looking at the
bottom.

4

Judgment and experience is necessary to decide how much
contamination' can be tolerated.


(b)

Acidity
The oxidation process in mineral oils is auto-catalytic, that is once
significant oxidation products have been formed they act as catalysts for
further oxidation and deterioration proceeds rapidly.
Determination of acidity (neutralization number) provides a useful guide.
For straight oils the new oil value will be less than 0.1 mg KOH/gm. The
end of the useful service life of an oil is indicated by an acid value of 2 - 3
mgKOH/gm; the value is not critical as sludge precipitation is not likely
until a value of 40-50 mg KOH/gm but deterioration proceeds rapidly once
a value of 2-3 mg KOH/gm has been reached and action at this level
provides a useful margin of safety. Little can be done to restore the
situation once this stage has been reached. Purging does not give much
advantage because the oxidation products remaining act as catalysts.
The situation with additive-containing oils is different. With oils containing
anti-oxidants, acidity development proceeds rapidly once the additive has
been reduced below a certain level. The normal recommendation is to
take 1 mg KOH/gm as an indication of the end of service life. It has
been found however that in certain systems where rapid depletion of antioxidant can take place (e.g. when the oil becomes contaminated with
oxidizing materials, as can happen on nitrous gas compressors or where
there is a buildup of soluble copper in the oil) it is useful to check the
antioxidant level and restore it to the new oil value by adding an antioxidant concentrate. In the case of systems prone to copper uptake, 8
ppm copper appears to be a critical level.
In certain additive-containing gear oils and high duty hydraulic oils, the
acidity of the new oil is influenced by the additives and may have values
ranging from 0.5 to 1.5 mg KOH/gm. In these oils the acidity first
decreases as the additives are depleted and then increases again as
deterioration occurs. The end of useful life is indicated when the acidity
rises to a value of 1 mg KOH/gm, though in such cases it is best to consult
the oil supplier for a more precise recommendation.

I


11.4
11.4.1

Contamination Problems
Contamination by Water

Free water in suspension at concentrations as low as 75 ppm can turn a light
colored, low viscosity oil at room temperature slightly hazy.
Oil in good condition may contain up to 200 ppm of water and remain clear.
Above this, water should quickly separate but if oils degrade or become
contaminated the rate of separation is reduced and even stable emulsions can
be formed. It is undesirable to operate with oils in this condition as corrosion may
occur and the corrosion products - iron hydrates - act as emulsion stabilizers.
If centrifuges or coalescers are fitted then these should be checked for
satisfactory operation; the usual faults are too low oil temperature in centrifuges
(the oil should be at 70 - 80°C) or binding in the case of coalescers through
failure to drain. Even without purification equipment, failure to drain settled water
from the oil tank may result in emulsion formation when the water is taken up by
the pump. When stable emulsions are formed and cannot be broken down the oil
should be changed as soon as possible.

11.4.2

Contamination with Air:

Most lubricated mechanisms tend to entrain bubbles of air in the oil. It is
important for the maintenance of satisfactory lubrication that the bubbles should
migrate to the surface and the foam formed there quickly collapse. Lubrication
systems are designed to allow these processes to occur. For example, oil drain
lines should have a gentle slope (I in 40) and run less than half full; the oil should
be returned to the reservoir surface and not dropped in from a height; the
reservoir residence time should be sufficient to allow complete separation (4-10
minutes).
Entrainment and foaming problems arise in lubrication systems either because
excessive air is entrained or the rate of foam collapse is too low. Entrainment can
arise in a number of ways; air leaks into the oil pump suction; rough edges to oil
delivery nozzles; splashing of return oil on the reservoir surface; whipping
of loose pieces on the surface of the oil. Slowing of bubble disengagement and
foam collapse is the result of degradation or contamination of the oil; it is rarely
possible to determine the cause by analysis as only very small amounts of the
contaminant need to be present.


In small systems the best approach is to change the oil; if this does not improve
the situation then it is necessary to look for the mechanical cause. When large
quantities of oil are involved comparative tests on the new oil and oil from
the system can be carried out to check whether the air release and foam collapse
properties have deteriorated. Method IP313 can be used for the air release;
Method IP146 for foam stability.
A summary of the routine analytical tests and the action level applicable to
normal industrial lubrication systems is given in Table 5.

TABLE 5 - SUMMARY OF ROUTINE ANALYTICAL TESTS FOR INDUSTRIAL
OILS

BIBL IOGRAPHY
1

M J Neale (Ed) Tribology Handbook, Butterworths 1973

2

Institute of Petroleum. Methods for Analysis and Testing for
Petroleum and its Products.


APPENDIX A
VISCOSITY EQUIVALENTS
This table may be used for approximate conversion from one viscosity scale to
another, at the same temperature.


APPENDIX B
SYMBOLS AND PREFERRED UNITS

1

Bulk Modules

E

Bar-1

2

Thermal Conductivity

K

W/M0C

3

Pressure

P

Bar

4

Solubility

S

-

5

Temperature

t

°c

6

Absolute Temperature

T

o

7

Volume

V

M3

8

Constants

a,b

9

Parameter

10

Density

Kg/L

11

Kinematic Viscosity

cSV

12

Dynamic Viscosity

cP

13

Specific Heat

kJ
Kg o C

K


FIGURE 1

LUBRICANT CHANGE PERIODS AND TESTS


FIGURE 2

CHARACTERISTICS OF MINERAL LUBRICATING OILS. VG 32
TO VG 460.


FIGURE 3

SERVICE MONITORING AND MAINTENANCE OF OIL IN
SERVICE ON LARGE SYSTEMS


DOCUMENTS REFERRED TO IN THIS ENGINEERING DESIGN GUIDE
This Engineering Design Guide makes reference to the following documents:
BRITISH STANDARDS
BS 489

Steam Turbine Oils (referred to in Clause 4.3)

BS 2626

Lubricating Oils for Refrigerant Compressors
(referred to in Clause 4.3)

BS 4231
(ISO 3448)

Classification for Viscosity Grades of Industrial Liquid
Lubricants (referred to in Clause 3.5).

BS 4475

Straight Mineral Lubricating Oils (referred to in
4.2, 7 and 9.1 and also Table 2).

SHW 295

Effect on the Skin of Mineral Oil (referred to in
Clause 10)

SHW 295A

Cancer of the Skin Caused by Mineral Oil (referred to in
Clause 10)

SHW 367

Dermatitis. A Cautionary Notice (referred to in Clause 10)

SHW 397

Cautionary Notes: Effects of Mineral Oil on Skin (referred to
in Clause 10).


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