viscosity - intro

8/7/2019 viscosity - intro

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VISCOSITY

INTRODUCTION

Viscosity is the measure of the internal friction in a liquid or the resistance to a

flow.

Low viscosity fluids flow easily (water, alcohol);High viscosity fluids pour slowly (molasses, cold honey, etc).

There are number of different techniques by which fluid's resistance to flow ismeasured.

Because Viscosity changes with temperature and sometimes also with pressure,it is also important that when different fluids are compared that themeasurements were conducted under the same temperature and pressure

conditions.

The common metric unit of absolute viscosity is the poise.

For convenience, the CentiPoise (cP) one one-hundredth of a poise is theunit customarily used.

Laboratory measurements of viscosity normally use the force of gravity toproduce flow through a capillary tube (viscometer) at a controlled temperature.This measurement is called kinematic viscosity.

The more customary unit is the centistoke (cSt) one one-hundredth of a stoke.

VISCOSITY - General

We are accustomed to the notion of friction as a force that is exerted opposite to that

which brings about motion when one solid moves in contact with another.

Such friction force tends to slow and eventually stop movement, unless the propulsive

force is maintained so that the friction force is equalized.

There is also a friction where solid moves through liquid, as when ship plows throughwater. The ship once set in motion will come to halt; unless the propulsive force is

maintained here too. Although water seems so smooth and lacking any projection to catch

the ship, water nevertheless absorbs enough energy when it is pushed apart by the ship to

eventually stop it.
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This friction arises from the fact that it is necessary to expend energy (power) to pull thewater apart against its own cohesive forces in order to make room for the ship to pass

through it.

The energy expanded varies greatly with the shape of the object that passes through the

fluid.

If the fluid is pulled apart gently and gradually, and then brought back even more gently

and gradually, then the energy needed to be expanded is minimal.

Such action is possible only by object that is of "teardrop" shape.

By contrast if the fluid is pulled apart abruptly in such a way as to force it into eddies andother turbulence, such as by cube, the maximum energy will have to be expanded to

move the cube through the fluid.

The friction between a moving solid and a surrounding liquid increases with velocity, sono matter how streamlined the object may be, eventually a terminal velocity will bereached and thus and object falling through the water accelerated by gravitational pull,

will eventually fall at a constant speed.

Any object will sink faster in fluid of low viscosity such as water, and will sink much

slower in a high viscosity fluid such as glycerin.

Viscosity of a fluid can thus be "measured" by the time it takes for object, such as a steel

ball bearing, fall through a test tube with oil, for example.

The friction makes itself evident even when the liquid itself is the only substanceinvolved. When any liquid moves, or pours, it does not move all-in-one as a solid does.

Instead, a given portion of the liquid will move relative to a neighboring portion and"internal friction" between these two portions will counter the motion. Where the

cohesive forces that impose this internal friction are low, as in water, we are not

ordinarily very aware of this. When the cohesive forces are high such as in cold honey,

the fluid pours very slowly.

The internal friction for any fluid is higher at low temperature, and much lower at higher

temperatures. So honey that was in refrigerator, will barely flow, but once warmed to

room temperature will pour easily.

The difference in flow between freezing cold and boiling hot water however is so small

as not being perceivable by human senses. However, very sophisticated laboratory

equipment can detect the difference.

Where the difference in flow between hot and cold fluid is very small, such fluid is said

to have HIGH VISCOSITY INDEX. (High VI)


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By contrast honey will be solid at freezing and water like at boiling temperatures, such

fluid is said to have LOW VISCOSITY INDEX. (Low VI)

At room temperature water has viscosity of just about ONE CentiPoise, while the

common anesthetic - diethyl ether has viscosity of 0.23 CentiPoise or 23 MilliPoise, and

glycerol is about 1,500 CentiPoise or 15 Poises.

The unit of viscosity the Poise has been named in honor of French physician Jean LouisPoiseuille (1799-1869), who in 1843 was the first to take time to study viscosity in

quantitative manner.

As a physician he was interested in the manner in which blood moved through blood

vessels. But his observations proved to be valid for ALL liquids.

For the purpose of lubrication, viscosity has been since the beginnings of Lubrication

Engineering held as the most important quality of the Lubricant.

The reason for this is that if the lubricant is too thin, it gets forced out from bearing

surface under pressure and poor lubrication occurs, this leads to bearing surface damage.

If the lubricant is too viscous, it either does not flow into the bearing surface, causing

lube starvation, and thus certain bearing damage. Or it consumes too much energy, which

is then converted to heat and the bearing may be overheated, at which point it can seize

due to loss of running clearance.

Therefore oil that is either too viscous or too thin, will cause premature failure of any

bearing surface.

The proper viscosity for given application is therefore extremely important.

That is why the first lubricant standard J300 that was developed by SAE in 1911 was

Viscosity Classification of Motor Oils, and although this standard was revised and

updated many times it is still used today world-wide for Motor Oil applications.

However unlike the exact scientific value ofPoise for Absolute Viscosity, the SAE

viscosity numbers are "staircase" approximations for KINEMATIC Viscosity.

For example Motor Oil that is measured to have viscosity of 9.5 cSt @ 100 C will be

rated as SAE 30, while another Motor Oil that is measured to have viscosity of 12 cSt ( or26% more viscous ) will also be rated as SAE 30 Motor Oil.

Yet in real life operation 26% difference in viscosity may make difference between

engine that will run forever and one that will wear out prematurely.

That is why "stay in grade" over the service life of the Motor Oil is so important!


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The SAE J300 measurement is only relating to a FRESH UNUSED MOTOR OIL.

As few as 20 hours of operation will change viscosity of pure petroleum oil.

Some oils will shear and thin out to SAE 20 or below, while some oils will oxidize and

sludge up to become much more viscous like SAE 40 or even SAE 60!

The "best" motor oil will be SAE 30 when fresh and SAE 30 when drained out after its

use.

Motorists make the common mistake that every SAE 30 oil is the same in performance,

but the reality, however, is quite different.

Even more drastic differences in viscosity for fresh and used oils can be observed in

multi-viscosity or multi-grade oils such as SAE 5W-30.

The SAE J300 viscosity classification therefore should not be confused with any level ofquality or long term performance. API Service Classifications are used to distinguish

Motor Oil performance levels and are based on specific engine and laboratory tests.

No matter what oil you use for any purpose the ideal viscosity that provides the ultimatelubrication, that is TOTAL bearing surface separation, and at MINIMUM power that is

consumed by the lubricants viscosity (MINIMUM TEMPERATURE RISE) occurs

ONLY at ONE combination of:

SPEED

LOAD

TEMPERATURE.

Under ALL other combinations of the three factors, the lubricant is NOT IDEAL.

Some lubricants, due to much higher than normal viscosity index, can have more

advantageous performance over much wider range of TEMPERATURE, SPEED and

LOAD, than others and therefore can be used more universally in wide range of

applications.

That is why some lubricants such as single grade SAE 30, must be changed to SAE 20when operating temperature is reduced or to SAE 40 or SAE 50 when the operating

temperature is increased.

So thicker more viscous oil is needed when engine is operated at higher temperature such

as high summer heat.

Similarly the proper viscosity depends on LOAD, the higher the load the thicker or

higher SAE number is required. So on highly loaded engine designed to used SAE 30 oil

under normal operation SAE 40 or SAE 50 should be utilized.


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Speed however has the opposite effect, when engine designed to run at 2,000 RPM isconstantly run at 6,000 RPM but at the same load, the SAE 30 oil should be substituted

with SAE 20 oil. Higher operating speed requires thinner or lower viscosity lubricant.

It is possible in some applications that the increase in load can be just offset by the

increase in speed and then the same oil such as SAE 30 that is just right for NORMALoperation will be also JUST RIGHT for the new HIGH LOAD and HIGH SPEED

regime.

"Old" truckers are well aware of this from experience, they get much better and longer

engine life when running in lower gear up-hill. Extra LOAD is imposed on the engine byclimbing uphill (lifting cargo weight against the pull of gravity requires more power

therefore the engine LOAD is increased), this can be balanced by running engine at much

higher RPM (this requires thinner lubricant).

The alternative of running uphill in low gear, that is at slow engine speed and increased

load would surely require increase in motor oil viscosity or else almost certain enginedamage would result.

It would be rather inconvenient to change motor oil before and after every major hill on

the Interstate. Therefore changing gears is much more feasible.

Thinner motor oils such as 5W-20 or even 0W-20 are becoming more popular these days

and are even specified by some OEM's (FORD & HONDA) on new 2001 cars.

Although these oils are promoted as "energy conserving" they generally trade a gain of

less than 0.1 MPG in Corporate Average Fuel Economy (CAFE) for shorter useful engine

life.

FORD which has previously designed cars to have 10 year or 150,000 miles life has

reduced the mileage life expectation to "beyond 100,000 miles" on vehicles that are

operated on SAE 5W-20 Motor Oil.

HONDA only claims "useful life" as 7-years or 70,000 miles in EPA certifications for

their CIVIC which uses SAE 5W-20 Motor Oil, while the previous model that utilized

SAE 5W-30 Motor Oil was certified for 10 year or 100,000 mile durability.

Since both HONDA and FORD Warranty their NEW cars for ONLY 3-years or 36,000-

miles the reduction in engine life expectancy is not a factor.

By contrast Mercedes-Benz recommends use of ONLY Synthetic Motor Oil that is at

least SAE 5W-40! This is a recent increase in recommended viscosity from SAE 5W-30.Apparently customer research indicated that engine longevity is more important to typical

MB customer than fuel economy.


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Even more important is the High-Shear High-Temperature MINIMUM specification inSAE J300. In tables below you will notice that there are "two" SAE 40 specifications,

one with minimum HSHT value of 2.9 cP for Automotive Oils (SAE 0W-40; 5W-40;

10W-40) and the other for Heavy Duty Oils (HDO) (SAE 15W-40; 20W-40; 25W-40;

40).

This double specification is at insistence of heavy duty engine manufacturers who have

required HSHT viscosity limits consistent with good engine durability in high-load,severe service operation. HSHT value of 3.7 cP or 27% more viscous oil at 150C

(300F).

Yes, a 27% increase in viscosity makes a difference between Automotive engine that

lasts 100,000 miles and Truck engine that lasts 1,000,000 miles!

When you consider that most Automotive Motor Oils are ONLY 3 cP, while our

SAE 5W-50SynLube Lube-4-LifeMotor Oil has rating of 5 cP, you can readily

appreciate why we can claim 300% to 500% increase in typical Automotive enginedurability, and that is with substantial "safety" reserve!

If you wish to learn more about viscosity, following definitions which are also mirrored

in our GLOSSARY should give you more technical know-how than you ever dreamed

possible!

Viscosity

The measure of the internal friction or the resistance to flow a liquid.Low viscosity fluids flow easily (water);High viscosity fluids pour slowly (molasses).

Measurement of a fluids resistance to flow. The common metric unit of absolute viscosity is thepoise, which is defined as the force in dynes required to move a surface one square centimeter inarea past a parallel surface at a speed of one centimeter per second, with the surfaces separatedby a fluid film one centimeter thick. For convenience, the CentiPoise (cP) one one-hundredthof a poise is the unit customarily used. Laboratory measurements of viscosity normally use theforce of gravity to produce flow through a capillary tube (viscometer) at a controlled temperature.This measurement is called kinematic viscosity. The unit of kinematic viscosity is the stoke,expressed in square centimeters per second. The more customary unit is the centistoke (cSt) one one-hundredth of a stoke. Kinematic viscosity can be related to absolute viscosity by theequation:

cSt = cP fluid density

In addition to kinematic viscosity, there are other methods for determining viscosity, including:
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Saybolt Universal viscosity

Saybolt Furol viscosity

Engler viscosity

Redwood viscosity.

Since viscosity varies inversely with temperature, its value is meaningless unlessthe temperature at which it is determined is reported.

See: viscosity index, viscosity-temperature relationship,

Absolute Viscosity

the ratio of shear stress to shear rate.It is a fluids internal resistance to flow.

The common unit of absolute viscosity is the poise and CentiPoise cP (see viscosity).

Absolute viscosity divided by the fluids density equals kinematic viscosity.

Absolute viscosity is typically measured by a rotary viscometers to determine the torque onrotating spindle and so measure the fluid's shear resistance. Changing the rotor (spindle)dimensions and the gap between the rotor and stator wall (container) and the speed of rotationcan change the rate of shear.

Examples of rotary viscometers that are used for Absolute Viscosity measurements:

Cold Cranking Simulator (C.C.S)

Mini-Rotary Viscometer (MRV)

Brookfield Viscometer

Tapered Bearing Simulator

In relation to oils for Automotive applications such as Motor Oil or Gear Oil, the CCS and MRVtest equipment at low temperatures is used to determine if the test lubricant does not get too thickto prevent safe engine or transmission operation at low temperatures.

If Motor Oil is too viscous to flow, even if engine can be started, certain mechanical damage willresult due to localized oil starvation. In transmissions both manual and automatic, proper shiftingmay be impaired, affecting safe vehicle operation once vehicle is put in motion.
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viscosity-temperature relationshipthe manner in which the viscosity of a given fluid varies inversely with temperature. Because ofthe mathematical relationship that exists between these two variables, it is possible to predictgraphically the viscosity of a petroleum fluid at any temperature within a limited range if theviscosities at two other temperatures are known. The charts used for this purpose are the ASTMStandard Viscosity-Temperature Charts for Liquid Petroleum Products, available in 6 ranges. Iftwo known viscosity-temperature points of a fluid are located on the chart and a straight linedrawn through them, other viscosity-temperature values of the fluid will fall on this line; however,values near or below the cloud point of the oil may deviate from the straight-line relationship.

Viscous

Possessing viscosity. From the Latin word for a sticky species of birdlime that is a slowly-pouringliquid.Frequently used to imply high viscosity.

viscometerdevice for measuring viscosity; commonly in the form of a calibrated capillary tube through whicha liquid is allowed to pass at a controlled temperature in a specified time period.

See kinematic viscosity, Saybolt Universal Viscosity.
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SAE

The Society of Automotive Engineers (SAE) is an engineering society founded to develop, collect,and disseminate knowledge of mobility technology.

SAE J300 Viscosity Classification (April 1997)

SAE

Viscosity

Grade

Low Temp.

Cranking

Low Temp.

Pumping

Minimum

Kinematic

Maximum

Kinematic

Hi-Temp. Hi-

Shear

0W 3,250 @ -30 60,000 @ -40 3.8

5W 3,500 @ -25 60,000 @ -35 3.8

10W 3,500 @ -20 60,000 @ -30 4.1

15W 3,500 @ -15 60,000 @ -25 5.6

20W 4,500 @ -10 60,000 @ -20 5.6

25W 6,000 @ -5 60,000 @ -15 9.3

20 5.6 9.3 2.6

30 9.3 12.5 2.9

40 12.5 16.3 2.9

40 12.5 16.3 3.7

50 16.3 21.9 3.7

60 21.9 26.1 3.7

5W-50* 3,500 @ -30 30,000 @ -40 16.5 20.0 5.0

*The SAE 5W-50 rating shown above is for SynLube Lube-4-LifeMotor Oil.

However, the previous specification has been revised by SAE in December 1999 to one

tabulated below.

According to "new" J300 our existing version ofSynLube Lube-4-LifeMotor Oilshould have been classified as SAE 0W-50, however our customer research has shown

that this unusual classification was "too radical" and "too scary", so we have decided toretain our existing rating ofSAE 5W-50 that was originated in 1985. This required slight

"thickening" of the lubricant at low temperatures, achieved by only 2% increase of one of

our existing ingredients. By "missing" the target SAE 0W low temperature viscosity by50 cP at -40C we can "legally" label our lubricant as SAE 5W-50, while for practical

purpose offer to our customers cold performance that "matches" SAE 0W motor oil.


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SAE J300 Viscosity Classification (Current)

SAE

ViscosityGrade

Low Temp.

Cranking

Low Temp.

Pumping

Minimum

Kinematic

Maximum

Kinematic

Hi-Temp. Hi-

Shear

0W6,200 @ -

3560,000 @ -40 3.8

5W6,600 @ -

3060,000 @ -35 3.8

10W7,000 @ -

2560,000 @ -30 4.1

15W7,000 @ -

2060,000 @ -25 5.6

20W9,500 @ -

1560,000 @ -20 5.6

25W13,000 @ -

1060,000 @ -15 9.3

20 5.6 9.3 2.6

30 9.3 12.5 2.9

40 12.5 16.3 2.9

40 12.5 16.3 3.7

50 16.3 21.9 3.7

60 21.9 26.1 3.7

5W-50*6,250 @ -

3530,000 @ -40 16.5 20.0 5.0

*The SAE 5W-50 rating shown above is for SynLube Lube-4-LifeMotor Oil.

ISO

International Standards Organization

This organization which is worldwide in scope sets standards and classifications for lubricants. Anexample is the ISO viscosity grade system.


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ISO v iscos i t y c lass i f ica t ion system

international system, approved by the International Standards Organization (ISO),for classifying industrial lubricants according to viscosity.

Each ISO viscosity grade number designation corresponds to the mid-pointof a viscosity range expressed in centistokes (cSt) at 40C.

For example:

lubricant with an ISO grade of 32 has a viscosity within the range of 28.8 35.2 cSt, the mid-point of which is 32. (see Table below)

ISO viscosity grade number Table:

viscosity range expressed in centistokes (cSt) at 40C

ISO #Mid-

PointMinimum Maximum

2 2.2 1.98 2.42

3 3.2 2.88 3.52

5 4.6 4.14 5.06

7 6.8 6.12 7.48

10 10 9.0 11.0

15 15 13.5 16.5

22 22 19.8 24.232 32 28.8 35.2

46 46 41.4 50.6

68 68 61.2 74.8

100 100* 90 110

150 150 135 165

220 220 198 242

320 320 288 352

460 460 414 506

680 680 612 748

1000 1000 900 1100

1500 1500 1350 1650

*The SAE 5W-50SynLube Lube-4-LifeMotor Oil is rated ISO VG 100.
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Which SAE rating is the best ?

A:SAE 0W-60 but it is not available, yet !

Theoretically the best possible SAE Viscosity rating is 0W-60, but only small experimentalquantities of such lubricants were ever produced. The NASASynLubeis rated SAE 0W-60,but it sells for $90.00 per Liter, therefore it is not economical or practical for average automotiveuse.

SynLube Lube-4-Lifeis rated SAE 5W-50 (ISO 100)

Below is the list of SAE Viscosity Ratings in order of preference from Best to Worst:

The Best Possible SAE 0W-60 (Very Expensive)

The Best Available SAE 5W-50 (Possible only with Fully Synthetic Motor Oil)

2nd Best SAE 5W-40 for Colder Climates (Synthetic or Blend)

SAE 10W-50 for Warmer Climates (Synthetic or Blend)

3rd Best SAE 5W-30 for Colder Climates

SAE 10W-40 for Normal Climates

SAE 20W-50 for Hot Climates**

Average SAE 10W-30

SAE 15W-40 for Heavy Duty Diesel Applications

Acceptable SAE 30 for Normal Climates, but not in Winter

SAE 40 for Warmer Climates, but not in Winter

SAE 50 for Hot Climates, but not in Winter

ter use only**

sense of the above recommendations we must define what all those climatic conditions

SAE 5W-20 for sub-zero temperatures, Win

Legend:** = unless use is prohibited by the engine manufacturer

To makemean. The definitions can be found in the table below:


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ClimaticCondition

ClimaticCode

Minimum Minimum Maximum Maximum Typical Typic

Low Low High High

. C

Coolant

Temp.

F

al

Coolant

Temp.

C

Ideal

SAE

ViscosityTemp. F Temp. C Temp. F Temp

Very Hot AA >80 >26 >110 >43 212 100 60

Hot A >60 >16 >110 >43 200 93 50

Warm B >50 >10 4 -7

viscosity - intro

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