Engineering Tribology 2011 Part 2 docx

2.6 VISCOSITY-SHEAR RATE RELATIONSHIPFrom the engineering view point, it is essential to know the value of the lubricant viscosity at a specific shear rate.. For simplicity it is usually

α is the pressure-viscosity coefficient [m2/N];

β is given by the following expression [12,13]:

β = [ln η 0 + 9.67] [ S [

( θ − 138) 0 [1 + 5.1× 10 p]− 9 Z 0

The above formula appears to be more comprehensive than the others since it takes intoaccount the simultaneous effects of temperature and pressure The ‘α’ values and dynamicviscosity ‘η0’ for some commonly used lubricants are given in Table 2.3 [12,14]

TABLE 2.3 Dynamic viscosity and pressure-viscosity coefficients of some commonly used

lubricants (adapted from [12])

2.6 VISCOSITY-SHEAR RATE RELATIONSHIP

From the engineering view point, it is essential to know the value of the lubricant viscosity

at a specific shear rate For simplicity it is usually assumed that the fluids are Newtonian, i.e.their viscosity is proportional to shear rate as shown in Figure 2.5

τ

Shear ratesα

tan α = η

u/h

FIGURE 2.5 Shear stress - shear rate characteristic of a Newtonian fluid

For pure mineral oils this is usually true up to relatively large shear rates of 105 - 10 6 [s -1] [31],but at the higher shear rates frequently encountered in engineering applications thisproportionality is lost and the lubricant begins to behave as a non-Newtonian fluid In thesefluids the viscosity depends on shear rate, that is the fluids do not have a single value ofviscosity over the range of shear rates Non-Newtonian behaviour is, in general, a function

of the structural complexity of a fluid For example, liquids like water, benzene and light oilsare Newtonian These fluids have a loose molecular structure which is not affected byshearing action On the other hand the fluids in which the suspended molecules form astructure which interferes with the shearing of the suspension medium are considered to benon-Newtonian Typical examples of such fluids are water-oil emulsions, polymer thickenedoils and, in extreme cases, greases The non-Newtonian behaviour of some selected fluids isshown in Figure 2.6

FIGURE 2.6 Viscosity - shear rate characteristics for some non-Newtonian fluids

There are two types of non-Newtonian behaviour which are important from the engineeringviewpoint: pseudoplastic and thixotropic behaviour

Pseudoplastic Behaviour

Pseudoplastic behaviour is also known in the literature as shear thinning and is associatedwith the thinning of the fluid as the shear rate increases This is illustrated in Figure 2.7

During the process of shearing in polymer fluids, long molecules which are randomlyorientated and with no connected structure, tend to align giving a reduction in apparentviscosity In emulsions a drop in viscosity is due to orientation and deformation of theemulsion particles The process is usually reversible Multigrade oils are particularlysusceptible to this type of behaviour; they shear thin with increased shear rates, as shown inFigure 2.8 [38].

τ

FIGURE 2.7 Pseudoplastic behaviour

The opposite phenomenon to pseudoplastic behaviour, i.e thickening of the fluid whenshear rate is increased, is dilatancy Dilatant fluids are usually suspensions with a high solidcontent The increase in viscosity with the shear rates is attributed to the rearranging of theparticles suspended in the fluid, resulting in the dilation of voids between the particles Thisbehaviour can be related to the arrangement of the fluid molecules The theory is that in thenon-shear condition molecules adopt a close packed formation which gives the minimumvolume of voids When the shear is applied the molecules move to an open pack formationdilating the voids As a result, there is an insufficient amount of fluid to fill the voids giving

an increased resistance to flow An analogy to such fluids can be found when walking on wetsand where footprints are always dry

υ

100 200 500

1 000

2 000

350 cS silicone SAE 30

Thixotropic Behaviour

Thixotropic behaviour, also known in the literature as shear duration thinning, is shown inFigure 2.9 It is associated with a loss of consistency of the fluid as the duration of shearincreases During the process of shearing, it is thought that the thixotropic fluids have astructure which is being broken down The destruction of the fluid structure progresses withtime, giving a reduction in apparent viscosity, until a certain balance is reached where thestructure rebuilds itself at the same rate as it is destroyed At this stage the apparent viscosityattains a steady value In some cases the process is reversible, i.e viscosity returns to itsoriginal value when shear is removed, but permanent viscosity loss is also possible

FIGURE 2.9 Thixotropic behaviour

A converse effect to thixotropic behaviour, i.e thickening of the fluid with the duration ofshearing, can also occur with some fluids This phenomenon is known in the literature asinverse thixotropy or rheopectic behaviour [19] An example of a fluid with such properties issynovial fluid, a natural lubricant found in human and animal joints It was found that theviscosity of synovial fluid increases with the duration of shearing [20,39] It seems that thelonger the duration of shearing the better the lubricating film which is generated by the body

Various viscosity measurement techniques and instruments have been developed over theyears The most commonly used in engineering applications are capillary and rotationalviscometers In general, capillary viscometers are suitable for fluids with negligible non-Newtonian effects and rotational viscometers are suitable for fluids with significant non-Newtonian effects Some of the viscometers have a special heating bath built-in, in order tocontrol and measure the temperature, so that the viscosity-temperature characteristics can beobtained In most cases water is used in the heating bath Water is suitable for thetemperature range between 0° to 99°C For higher temperatures mineral oils are used and forlow temperatures down to -54°C, ethyl alcohol or acetone is used

Capillary Viscometers

Capillary viscometers are based on the principle that a specific volume of fluid will flowthrough the capillary (ASTM D445, ASTM D2161) The time necessary for this volume of

fluid to flow gives the ‘kinematic viscosity’ Flow through the capillary must be laminar and

the deductions are based on Poiseuille’s law for steady viscous flow in a pipe There is anumber of such viscometers available and some of them are shown in Figure 2.10

Assuming that the fluids are Newtonian, and neglecting end effects, the kinematic viscositycan be calculated from the formula:

υ = πr 4 glt / 8LV = k(t 2 − t 1 ) (2.15)where:

υ is the kinematic viscosity [m2/s];

r is the capillary radius [m];

l is the mean hydrostatic head [m];

g is the earth acceleration [m/s2];

L is the capillary length [m];

V is the flow volume of the fluid [m3];

t is the flow time through the capillary, t = (t 2 − t 1 ), [s];

k is the capillary constant which has to be determined experimentally by applying

a reference fluid with known viscosity, e.g by applying freshly distilled water.The capillary constant is usually given by the manufacturer of the viscometer

Capillary tube

Etched

rings

British Standard U-tube viscometer

Capillary tube Capillary

tube

Etched rings

Glass strengthening bridge

Kinematic viscometers

for transparent fluids for opaquefluids

FIGURE 2.10 Typical capillary viscometers (adapted from [23])

In order to measure the viscosity of the fluid by one of the viscometers shown in Figure 2.10,the container is filled with oil between the etched lines The measurement is then made bytiming the period required for the oil meniscus to flow from the first to the second timingmark This is measured with an accuracy to within 0.1 [s]

Kinematic viscosity can also be measured by so called ‘short tube’ viscometers In theliterature they are also known as efflux viscometers As in the previously describedviscometers, viscosity is determined by measuring the time necessary for a given volume offluid to discharge under gravity through a short tube orifice in the base of the instrument.The most commonly used viscometers are Redwood, Saybolt and Engler The operationprinciple of these viscometers is the same, and they only differ by the orifice dimensions andthe volume of fluid discharged Redwood viscometers are used in the United Kingdom,Saybolt in Europe and Engler mainly in former Eastern Europe The viscosities measured bythese viscometers are quoted in terms of the time necessary for the discharge of a certainvolume of fluid Hence the viscosity is sometimes found as being quoted in Redwood and

Saybolt seconds The viscosity measured on Engler viscometers is quoted in Engler degrees,which is the time for the fluid to discharge divided by the discharge time of the same volume

of water at the same temperature Redwood and Saybolt seconds and Engler degrees caneasily be converted into centistokes as shown in Figure 2.11 These particular types ofviscometers, are gradually becoming obsolete, since they do not easily provide calculableviscosity A typical short tube viscometer is shown in Figure 2.12

In order to extend the range of kinematic, Saybolt Universal, Redwood No 1 and Englerviscosity scales only (Figure 2.11), a simple operation is performed The viscosities on thesescales which correspond to the viscosity between 100 and 1000 [cS] on the kinematic scale aremultiplied by a factor of 10 and this gives the required extension For example:

4000 [cS] = 400 [cS]×10≈1850 [SUS]×10 = 18500 [SUS]≈51 [Engler]×10 = 510 [Engler]

2 2.5 3 3.5 4 4.5 5 6 7 8 10

15 20 25 30 35 40 50 60 70 80 90 100

150 200 250 300 350 400 500 600 700 800

1 000

2 2.5 3 3.5 4 4.5 5 6 7 8 10

15 20 25 30 35 40 50 60 70 80 90 100

150 200 250 300 350 400 500 600 700 800

1 000

100

150 200 250 300 350 400 450 600 700 800 900

60 70 80 90

100

150 200 250 300 350 400 450 600 700 800

60 70 80 90

2 2.5 3 3.5 4 4.5 5

6 7 8 9 10

15 20 25 30 35 40 50 60 70 80

1.9 1.7 1.6

1.5 1.4 1.3 1.2

25 30 35 40 50 60 70 80 90 100

150 200 250 300 350 400 450

30 35 40 45 60 70 80 90 100

150 200 250 300 350 400 100

in between The measurements are conducted by applying either a constant torque andmeasuring the changes in the speed of rotation or applying a constant speed and measuring

the changes in the torque These viscometers give the ‘dynamic viscosity’ There are two

main types of these viscometers: rotating cylinder and cone-on-plate viscometers

Stopper

Capillary tube

Lubricant sample

Water bath

Overflow rim

FIGURE 2.12 Schematic diagram of a short tube viscometer

· Rotating Cylinder Viscometer

The rotating cylinder viscometer, also known as a ‘Couette viscometer’, consists of twoconcentric cylinders with an annular clearance filled with fluid as shown in Figure 2.13 Theinside cylinder is stationary and the outside cylinder rotates at constant velocity The forcenecessary to shear the fluid between the cylinders is measured The velocity of the cylindercan be varied so that the changes in viscosity of the fluid with shear rate can be assessed Careneeds to be taken with non-Newtonian fluids as these viscometers are calibrated forNewtonian fluids Different cylinders with a range of radial clearances are used for differentfluids For Newtonian fluids the dynamic viscosity can be estimated from the formula:

where:

η is the dynamic viscosity [Pas];

r b , r c are the radii of the inner and outer cylinders respectively [m];

M is the shear torque on the inner cylinder [Nm];

ω is the angular velocity [rad/s];

d is the immersion depth of the inner cylinder [m];

k is the viscometer constant, supplied usually by the manufacturer for each pair of

cylinders [m-3]

When motor oils are used in European and North American conditions, the oil viscositydata at -18°C is required in order to assess the ease with which the engine starts A speciallyadapted rotating cylinder viscometer, known in the literature as the ‘Cold CrankingSimulator’ (CCS), is used for this purpose (ASTM D2602) The schematic diagram of thisviscometer is shown in Figure 2.14

Fluid sample

ω

rc

r b

Inner cylinder (stationary)

Outer cylinder (rotating)

FIGURE 2.13 Schematic diagram of a rotating cylinder viscometer

Overload clutch

Constant-power motor drive with tachometer

Coolant

(methanol)

in

Coolant out

Nylon block

Thermocouple

ω

Lubricant sample

Rotating cylinder

Stationary cylinder

FIGURE 2.14 Schematic diagram of a cold cranking simulator

The inner cylinder is rotated at constant power in the cooled lubricant sample of volumeabout 5 [ml] The viscosity of the oil sample tested is assessed by comparing the rotationalspeed of the test oil with the rotational speed of the reference oil under the same conditions.The measurements provide an indication of the ease with which the engine will turn at lowtemperatures and with limited available starting power In the case of very viscous fluids,two cylinder arrangements with a small clearance might be impractical because of the veryhigh viscous resistance; thus a single cylinder is rotated in a fluid and measurements arecalibrated against measurements obtained with reference fluids

· Cone on Plate Viscometer

The cone on plate viscometer consists of a conical surface and a flat plate Either of thesesurfaces can be rotated The clearance between the cone and the plate is filled with the fluid

and the cone angle ensures a constant shear rate in the clearance space The advantage of thisviscometer is that a very small sample volume of fluid is required for the test In some ofthese viscometers, the temperature of the fluid sample is controlled during tests This isachieved by circulating pre-heated or cooled external fluid through the plate of theviscometer These viscometers can be used with both Newtonian and non-Newtonian fluids

as the shear rate is approximately constant across the gap The schematic diagram of thisviscometer is shown in Figure 2.15

The dynamic viscosity can be estimated from the formula:

where:

η is the dynamic viscosity [Pas];

r is the radius of the cone [m];

M is the shear torque on the cone [Nm];

ω is the angular velocity [rad/s];

α is the cone angle [rad];

k is the viscometer constant, usually supplied by the manufacturer [m-3]

Cone

Driving motor

Torque spring

Plate

α

Test fluid

The dynamic viscosity can be estimated from the formula:

η = 2r 2 (ρb − ρ)gF / 9v (2.18)where:

η is the dynamic viscosity [Pas];

r is the radius of the ball [m];

ρb is the density of the ball [kg/m3];

ρ is the density of the fluid [kg/m3];

g is the gravitational constant [m/s2];

v is the velocity of the ball [m/s];

F is the correction factor

Liquid level

Small hole

Sphere

Guide tube

Glass tube

Water bath

Timing marks Start

Stop

FIGURE 2.16 Schematic diagram of a ‘Falling Ball Viscometer’

The correction factor can be calculated from the formula given by Faxen [19]:

where:

d is the diameter of the ball [m];

D is the internal diameter of the tube [m]

There are also many other more specialized viscometers designed to perform viscositymeasurements, e.g under high pressures, on very small volumes of fluid, etc They aredescribed in more specialized literature [e.g 21]

In industrial practice it might be necessary to mix two similar fluids of different viscosities in

order to achieve a mixture of a certain viscosity The question is, how much of fluid ‘A’

should be mixed with fluid ‘B’ This can simply be worked out by using ASTM viscosity

paper with linear abscissa representing percentage quantities of each of the fluids Theviscosity of each of the fluids at the same temperature is marked on the ordinate on each side

of the graph as shown in Figure 2.17 A straight line is drawn between these points andintersects a horizontal line which corresponds to the required viscosity A vertical line drawnfrom the point of intersection crosses the abscissa, indicating the proportions needed of thetwo fluids In the example of Figure 2.17, 20% of the less viscous component is mixed with80% of the more viscous component to give the ‘required viscosity’

FIGURE 2.17 Determining the viscosity of a mixture

There are several widely used oil viscosity classifications The most commonly used are SAE(Society of Automotive Engineers), ISO (International Organization for Standardization) andmilitary specifications

SAE Viscosity Classification

The oils used in combustion engines and power transmissions are graded according to SAEJ300 and SAE J306 classifications respectively A recent SAE classification establishes elevenengine oil and seven transmission oil grades [34,35] The engine oil viscosities for differentSAE grades are shown in Table 2.4

Note that the viscosity in column 2 (Table 2.4) is the dynamic viscosity while column 3shows the kinematic viscosity The low temperature viscosity was measured by the ‘cold-cranking simulator’ and is an indicator of cold weather starting ability The viscositymeasurements at 100°C are related to the normal operating temperature of the engine The

oils without a ‘W’ suffix are called ‘monograde oils’ since they meet only one SAE grade The oils with a ‘W’ suffix, which stands for ‘winter’, have good cold starting capabilities For

climates where the temperature regularly drops below zero Celsius, engine and transmissionoils are formulated in such a manner that they give low resistance at start, i.e their viscosity

is low at the starting temperature Such oils have a higher viscosity index, achieved by

adding viscosity improvers (polymeric additives) to the oil and are called ‘multigrade oils’.

For example, SAE 20W/50 has a viscosity of SAE 20 at -18°C and viscosity of SAE 50 at 100°C

as is illustrated in Figure 2.18 The problem associated with the use of multigrade oils is thatthey usually shear thin, i.e their viscosity drops significantly with increased shear rates due

to polymeric additives added This has to be taken into account when designing machine

components lubricated by these oils The drop in viscosity can be significant, and with someviscosity improvers even a permanent viscosity loss at high shear rates may occur due to thebreaking up of molecules into smaller units The viscosity loss affects the thickness of thelubricating film and subsequently affects the performance of the machine.

TABLE 2.4 SAE classification of engine oils [34]

 SAE 30



SAE 20

 SAE 10

FIGURE 2.18 Viscosity-temperature graph for some monograde and multigrade oils (not to

scale, adapted from [12])

SAE classification of transmission oils is very similar to that of engine oils The onlydifference is that the winter grade is defined by the temperature at which the oil reaches the

viscosity of 150,000 [cP] This is the maximum oil viscosity which can be used withoutcausing damage to gears The classification also permits multigrading The transmission oilviscosities for different SAE grades are shown in Table 2.5 [35].

TABLE 2.5 SAE classification of transmission oils [35]

FIGURE 2.19 Comparison of SAE grades of engine and transmission oils

ISO Viscosity Classification

The ISO (International Standards Organization) viscosity classification system was developed

in the USA by the American Society of Lubrication Engineers (ASLE) and in the UnitedKingdom by The British Standards Institution (BSI) for all industrial lubrication fluids It isnow commonly used throughout industry The industrial oil viscosities for different ISOviscosity grade numbers are shown in Table 2.6 [36] (ISO 3448)

Lubricant density is important in engineering calculations and sometimes offers a simpleway of identifying lubricants Density or specific gravity is often used to characterize crudeoils It gives a rough idea of the amount of gasoline and kerosene present in the crude Theoil density, however, is often confused with specific gravity

Specific gravity is defined as the ratio of the mass of a given volume of oil at temperature ‘t 1’

to the mass of an equal volume of pure water at temperature ‘t’ (ASTM D941, D1217, D1298)

TABLE 2.6 ISO classification of industrial oils [36].

Kinematic viscosity limits [cSt] at 40°C

ISO viscosity grade

Density, on the other hand, is the mass of a given volume of oil [kg/m3]

In the petroleum industry an API (American Petroleum Institute) unit is used which is aderivative of the conventional specific gravity The API scale is expressed in degrees which insome cases are more convenient to use than the specific gravity readings The API specificgravity is defined as [23]:

where:

s is the specific gravity at 15.6°C (60°F)

As mentioned already the density of a typical mineral oil is about 850 [kg/m3] and, since thedensity of water is about 1000 [kg/m3], the specific gravity of mineral oils is typically 0.85

The most important thermal properties of lubricants are specific heat, thermal conductivityand thermal diffusivity These three parameters are important in assessing the heating effects

in lubrication, i.e the cooling properties of the oil, the operating temperature of the surfaces,etc They are also important in bearing design

Specific Heat

Specific heat varies linearly with temperature and rises with increasing polarity or hydrogenbonding of the molecules The specific heat of an oil is usually half that of water For mineraland synthetic hydrocarbon based lubricants, specific heat is in the range from about 1800[J/kgK] at 0°C to about 3300 [J/kgK] at 400°C For a rough estimation of specific heat, thefollowing formula can be used [5]:

σ = (1.63 + 0.0034θ) / s 0 5 (2.21)where:

σ is the specific heat [kJ/kgK];

θ is the temperature of interest [°C];

s is the specific gravity at 15.6°C

Thermal Conductivity

Thermal conductivity also varies linearly with the temperature and is affected by polarityand hydrogen bonding of the molecules The thermal conductivity of most of the mineraland synthetic hydrocarbon based lubricants is in the range between 0.14 [W/mK] at 0°C to0.11 [W/mK] at 400°C For a rough estimation of a thermal conductivity the followingformula can be used [5]:

where:

K is the thermal conductivity [W/mK];

θ is the temperature of interest [°C];

s is the specific gravity at 15.6°C

χ is the thermal diffusivity [m2/s];

K is the thermal conductivity [W/mK];

ρ is the density [kg/m3];

σ is the specific heat [J/kgK]

The values of density, specific heat, thermal conductivity and thermal diffusivity for sometypical materials are given in Table 2.7

The temperature characteristics are important in the selection of a lubricant for a specificapplication In addition the temperature range over which the lubricant can be used is ofextreme importance At high temperatures, oils decompose or degrade, while at lowtemperatures oils may become near solid or even freeze Oils can be degraded by thermaldecomposition and oxidation During service, oils may release deposits and lacquers oncontacting surfaces, form emulsions with water, or produce a foam when vigorouslychurned These effects are undesirable and have been the subject of intensive research Thedegradation of oil does not just affect the oil, but more importantly leads to damage of thelubricated equipment It may also cause detrimental secondary effects to the operatingmachinery A prime example of secondary damage is corrosion caused by the acidity ofoxidized oils The most important thermal properties of a lubricant are its pour point, flash

point, volatility, oxidation and thermal stability, surface tension, neutralization number andcarbon residue.

TABLE 2.7 Density, specific heat, thermal conductivity and thermal diffusivity values for

some typical materials

Pour Point and Cloud Point

The pour point of an oil (ASTM D97, D2500) is the lowest temperature at which the oil willjust flow when it is cooled In order to determine the pour point the oil is first heated toensure solution of all ingredients and elimination of any influence of past thermaltreatment It is then cooled at a specific rate and, at decrements of 3°C, the container is tilted

to check for any movement The temperature 3°C above the point at which the oil stopsmoving is recorded as the pour point This oil property is important in the lubrication of anysystem exposed to low temperature, such as automotive engines, construction machines,military and space applications When oil ceases to flow this indicates that sufficient waxcrystallization has occurred or that the oil has reached a highly viscous state At this stagewaxes or high molecular weight paraffins precipitate from the oil The waxes form theinterlocking crystals which prevent the remaining oil from flowing This is a critical pointsince the successful operation of a machine depends on the continuous supply of oil to themoving parts The viscosity of the oil at the pour point is usually very large, i.e severalhundred [Pas] [24], but the exact value is of little practical significance since what is important

is the minimum temperature at which the oil can be used

The cloud point is the temperature at which paraffin wax and other materials begin toprecipitate The onset of wax precipitation causes a distinct cloudiness or haze visible in thebottom of the jar This occurrence has some practical applications in capillary or wick fedsystems in which the forming wax may obstruct the oil flow It is limited only to thetransparent fluids since measurement is based purely on observation If the cloud point of an

oil is observed at a temperature higher than the pour point, the oil is said to have a ‘Wax Pour Point’ If the pour point is reached without a cloud point the oil shows a simple

‘Viscosity Pour Point’.

There is also another critical temperature known as the ‘Flock Point’, which is primarily

limited to refrigerator oils It is the temperature at which the oil separates from the mixturewhich consists of 90% refrigerant and 10% oil The Flock point provides an indication of howthe oil reacts with a refrigerant, such as Freon, at low temperature

Flash Point and Fire Point

The ‘flash point’ of the lubricant is the temperature at which its vapour will ignite In order

to determine the flash point the oil is heated at a standard pressure to a temperature which isjust high enough to produce sufficient vapour to form an ignitable mixture with air This isthe flash point The ‘fire point’ of an oil is the temperature at which enough vapour isproduced to sustain burning after ignition The schematic diagram of a flash and fire pointapparatus is shown in Figure 2.20

Oil bath

Pilot flame Gas

supply

Bath thermometer

Cup thermometer Stirrer

Test fluid

Gas

Cup thermometer

Closed-cup test apparatus Open-cup test apparatus

Test fluid

FIGURE 2.20 Schematic diagram of the flash and fire point apparatus

Flash and fire points (ASTM D92, D93, D56, D1310) are very important from the safety viewpoint since they constitute the only factors which define the fire hazard of a lubricant Ingeneral, the flash point and fire point of oils increase with increasing molecular weight For atypical lubricating oil, the flash point is about 210°C whereas the fire point is about 230°C

Volatility and Evaporation

In many applications the loss of lubricant due to evaporation can be significant Thetemperature has a controlling influence At elevated temperatures in particular, oils maybecome more viscous and greases tend to stiffen and eventually dry out because ofevaporation Volatile components of the lubricant may be lost through evaporation resulting

in a significant increase in viscosity and a further temperature rise due to higher frictionwhich causes further oil losses due to evaporation Volatility of lubricants is expressed as adirect measure of evaporation losses (ASTM D2715) In order to determine the lubricantvolatility, a known quantity of lubricant is exposed in a vacuum thermal balance device Theevaporated material is collected on a condensing surface and the decreasing weight of theoriginal material is expressed as a function of time Depending on available equipment it is

possible to obtain quantitative evaporation data together with some information on theidentity of the volatile products Frequently the evaporation rates are determined at varioustemperatures The schematic diagram of the evaporation test apparatus is shown in Figure2.21.

In this device a known quantity of oil is placed in a specially designed cup The air enters theperiphery of the cup and flows across the surface of the sample and exits through thecentrally located tube Prior to the test the cell is preheated to the required temperature in anoil bath The flow rate of air is about 2 [litres/min] The cup is aerated for 22 hours thencooled and weighed at the end of the test The percentage of lost mass gives the evaporationrate

Flow-controlled air supply

temperature

Constant-Test

fluid

Air-tight seal

Test cup

Oxidation of oils is a complex process Different compounds are being generated at differenttemperatures For example, at about 150°C organic acids are produced whereas at highertemperatures aldehydes are formed [24] The oxidation rates vary between differentcompounds, as shown in the frame below

Paraffins Naphthenes Aromatics

Most resistant

Asphaltenes

One way of improving oxidation stability is to remove the hydrocarbon type aromatics andmolecules containing sulphur, oxygen, nitrogen, etc This is achieved through refining Morerefined oil has better oxidation stability It is also more expensive and has poorer boundarylubrication characteristics, so the oil selection for a particular application is always acompromise, depending on the type of job the oil is expected to perform Oxidation can also

be controlled by additives which attack the hyperoxides formed in the initial stages ofoxidation or break the chain reaction mechanism by scavenging free radicals The products ofoxidation usually consist of acidic compounds, sludge and lacquers All of these compoundscause oil to become more corrosive, more viscous and also cause the deposition of insolubleproducts on working surfaces, restricting the flow of oil in operating units This interfereswith the performance of the unit Oxidation stability is a very important oil characteristic,especially where extended life is required, e.g turbines, transformers, hydraulic and heattransfer units, etc A lubricant with limited oxidation stability requires more frequentmaintenance or replacement resulting in higher operating costs Under more severeconditions the required oil changes may become more frequent, hence the operating costswill even be higher Many tests have been devised to assess the oxidation characteristics ofoils and there is no clear rationale for selecting a particular test [32] Some of them have beendevised for specific applications, for example, the assessment of oxidation characteristics ofrailway diesel engine lubricants [27] In most test apparatus the oil is in contact with selectedcatalysts and is exposed to air or oxygen and the effects are measured in terms of acid orsludge formed, viscosity change, etc A schematic diagram of a typical oxidation apparatus isshown in Figure 2.22

In this apparatus oxygen is passed through the oil sample placed in the reaction vessel Thereaction vessel consists of a large test tube with a smaller central removable oxygen inlet tubewhich supports the steel-copper catalyst coil At the end of the tube there is a water cooledcondenser which returns the more volatile components to the reaction About 300 [ml] of oiltogether with 60 [ml] of distilled water is placed in the test tube The flow rate of oxygen isabout 0.5 [litre/min] and the test is conducted at a temperature of 95°C During the test acidiccompounds are produced in the tube, and the neutralization number determined at the end

of the test is a measure of oxidation stability of the oil The tests are usually run over aspecific period of time It has to be mentioned, however, that the ASTM oxidation tests arestill under revision [28] and new techniques are being developed For example, DifferentialScanning Calorimetry has been employed to assess the oxidation stability of oils [e.g 40-44]

Thermal Stability

When heated above a certain temperature oils will start to decompose, even if no oxygen ispresent Thermal stability is the resistance of the lubricant to molecular breakdown ormolecular rearrangement at elevated temperatures in the absence of oxygen When heatedmineral oils break down to methane, ethane and ethylene Thermal stability can beimproved by the refining process, but not by additives It can be measured by placing the oil

in a closed vessel with a manometer monitoring the rate of pressure increase when thecontainer is heated at a specific rate under nitrogen atmosphere Mineral oils with a

substantial percentage of C-C single bonds have a thermal stability limit of about 350°C.

Synthetic oils, in general, exhibit better oxidation stability than mineral oils However therecan be exceptions For example, synthetic hydrocarbons produced by the polymerization oroligomerization process, although possessing the same basic structures as mineral oils, have

a thermal stability limit 28°C or more below that of mineral oils [22] Lubricants witharomatic linkages or with aromatic linkages and methyl groups as side chains exhibit athermal stability limit of about 460°C The additives used for lubrication improvementusually have a thermal stability below that of base oils In general, thermal degradation of theoil takes place at much higher temperatures than oxidation Thus the maximum

Oil sample

300 ml

Water

60 ml

Condenser jacket

Condenser water in

Condenser water out

Dried oxygen in (controlled pressure

& volume flow-rate) Used oxygen escapes past condenser

Catalyst coils:

steel and copper

wires

temperature bath at 95°C

Constant-Glass oxidation cell

FIGURE 2.22 Schematic diagram of the oxidation test apparatus [23]

temperature at which an oil can be used is determined by its oxidation stability In Figures2.23 and 2.24 the relationships between lubricant life and temperature are shown for mineraland synthetic oils respectively [29]

Surface Tension

Various lubricants generally show some differences in the degree of wetting and spreading

on surfaces Furthermore even the same lubricant can show different wetting and spreadingcharacteristics depending on the degree of oxidation or on the modification of the lubricant

by additives The phenomena of wetting and spreading are dependent on surface tension(ASTM D971, D2285) which is especially sensitive to additives, e.g less than 0.1 wt% ofsilicone in mineral oil will reduce the surface tension of the oil to that of silicone [22] Surfaceand interfacial tension are related to the free energy of the surface, and the attraction betweenthe surface molecules is responsible for these phenomena Surface tension refers to the freeenergy at a gas-liquid interface, while interfacial tension takes place at the interface betweentwo immiscible liquids Surface tension can be measured by the du Noy ring method (ASTMD971) The schematic diagram of surface tension measurement principles is shown in Figure2.25 It involves the measurement of the force necessary to detach the platinum wire ring

Thermal stability limit

(insignificant oxygen present)

Upper limit imposed by oxidation

where the oxygen supply is unlimited

Oils without anti-oxidants

Oils containing anti-oxidants

Life in this region depends on the amount of oxygen present



and the presence or absence of catalysts



Lower temperature limit imposed by the pour point

which varies with oil, source, viscosity, treatment & additives

Thermal stability limit for polyphenyl ethers

Oxidation limit for polyphenyl ethers

Thermal stability limit for silicones



Oxidation limit for esters and silicones



Thermal and oxidative limit

for phosphate esters Pour point limit for

FIGURE 2.24 Temperature-life limits for selected synthetic oils [29]

from the surface of the liquid The surface tension is then calculated from the followingformula [22]:

ring

r

FIGURE 2.25 Schematic diagram of surface tension measurement principles

Typical values of surface tension for some basic fluids are shown in Table 2.8 [22] Surfacetension is frequently used together with the neutralization number as a criterion for the oildeterioration in transformers, hydraulic systems and turbines Interfacial tension betweentwo immiscible liquids is approximately equal to the difference in the surface tensionbetween the two liquids

TABLE 2.8 Surface tension of some basic fluids [22]

method for determining the acidic condition of the oil The results are reported as a Total Acid Number (TAN) for acidic oils and as a Total Base Number (TBN) for alkaline oils TAN