9 - Paul B AMS_Presentation_1!4!2012

8/12/2019 9 - Paul B AMS_Presentation_1!4!2012

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Introduction:

Rolling element bearings are key components to many things that move.

They are used for power tools, electric motors, computer hard drives

and numerous satellite systems. However, the success of most bearing applications

is predicated on the suitability of the lubricant for the task at hand.

Rather than a sterile lecture on lubricant and tribological fundamentals, Im going to

take you with me through the process of formulating and testing a synthetic grease

intended for rolling element bearings. This process will hopefully distill 30 + years

of experience and hard work into a meaningful lecture!


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The case for synthetic lubricants versus petroleum depends on the following

chemical and physical factors:

Narrow Molecular Weight Distribution

Improved Thermooxidative Stability

Less change in viscosity as a function of temperature > VI

Superior Low Temperature Performance

PFPEs are inert towards oxygen, acids, bases and are non-flammable

Lot homogeneity!

Lower Vapor Pressure


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MW distribution refers to difference between the lowest viscosity and highest viscosity

fraction within a given grade of oil.

Petroleum Fluid

Synthetic

Molecular Weight

Frequency

If we desire to formulate a lubricant for an aerospace bearing application,


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which fluid would be more desirable and why?

Volatility

KV changes as a function of weight loss and its impact on drag for

bearings operating with a severely limited energy budget.

Thermooxidation refers to the combined ravages of heat and oxygen in

destroying lubricant molecules. In hard vacuum, oxidative degradationmay be neglected, but too much heat will degrade even the most robust

synthetic lubricants. Conservative approximations are:

Synthetic Hydrocarbons 150C

Synthetic Esters 200C temperature depends on duration

PFPEs 300C

Technical caveat is that at these temperatures low viscosity lubricantscan evaporate!


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Another important consideration is the change in a oils viscosity as a

function of temperature. All fluids become more viscous as temperatures decline and

decrease in viscosity as temperatures rise. The dimensionless parameter used to

describe this phenomenon is Viscosity Index.

For numerous application, fluids with a high VI are desirable since we can expect

Less of a change in film thickness as temperatures vary.

The VI of fluids can range from < 0 to greater than 600

SHCs are about 130

Esters are about 150

Linear PFPEs >300

600 = phenylmethylpolysiloxane not a good lubricant for metal/metal combinations


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Temperature, C

KV

VI is determined by measuring the kinematic viscosity at 40 and 100C

Plot of VI for imaginary petroleum and PAO fluid


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Since we have a rational for selecting a synthetic oil for a space bearing application,

what would a viable formulation?

Base Fluid: Polyalphaolefin e.g. PAO-6 KV 40 = 32 cSt KV100 = 5.8 cSt

Antioxidant: Phenolic AO appears to be the heritage AO 0.3% w/w

Boundary Additive: Phosphate Ester 1% w/w

The function of the AO is to retard oxidation of the base oil during long term

terrestrial storage.

The phosphate ester reduces metal damage when speeds, loads, and temperature all

conspire to induce asperity contact through the formation of a metallo-organic film.

Some chemical wear is preferable to the more damaging mechanical wear

This phenomenon is investigated by 4 ball tribometry per ASTM D4172/D2266


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Processing of the fluid would consist of heating with constant mechanical

agitation. This would usually be followed by ultrafiltration through a 0.45 micron

filter.

After ultrafiltration, the oils chemical and physical properties are determined.

LUBRICATING GREASES:

All lubricating greases contain three fundamental ingredients:

A base fluid that carries most of the tribological responsibility and usually consistingof 90+% of the composition for an NLGI Grade 2 grease.

A solid thickening agent that is used to immobilize the fluid.

Additives that confer certain desirable attributes, AOs AW, EP, RI etc.


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The chemical composition of oils and greases is determined by FT-IR.

Fourier Transform Infrared Analysis is a powerful analytical technique that

allows the rapid determination of the chemical nature of the base oil, the thickener

and additives. Figure 1 is the FT-IR spectrum of a lithium 12-OH stearate grease

Figure 1

Decreasing energyE = hc/


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The absorption bands or peaks are characterized as follows:

2955, 2921 and 2851 cm-1

are due to C-H bond stretching of the hydrocarbon molecules.

Bands at 1453 and 1376 are also due to the base oil and are the result of a less energetic

molecular choreography i.e. rocking, bending, waving.

The peak at 1580 reciprocal centimeters is the result of the thickener in the grease.

And the band at 962 cm-1identifies the phosphate ester AW agent.


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In Figure 2 we have exploded the region of the spectrum where the thickener resides

to illustrate a further benefit of FT-IR analysis.

Figure 2

If the Y-axis is absorbance, then peak intensity is a linear function of analyte

concentration. Therefore, the height of the peak at 1580 cm-1 is related to

the amount of thickener in the grease.


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The ability to determine the thickener concentration initially and after a period of

use in the field provides a powerful means of determining the suitability of

the grease for continued use.

It also important to realize that very small amounts of grease is needed for the FT-IR

analysis. Figure 3 shows an FT-IR equipped with an ATR attachment.


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Prototype rolling element bearing grease typical properties

Property Method ResultThickener Report Lithium Complex

Base Fluid Report Syn. Hydrocarbon

KV100C ASTM D445 13 cSt

Flash Point ASTM D92 >250C

Pour Point ASTM D97 -40C

Color Visual Amber

Po ASTM D217 234

P60 ASTM D217 243

Oil Separation24h at 100C

ASTM D6184 1.9%

Evaporation

24h at 100C

CTM 0.4%


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The most important property of any lubricant is viscosity. Viscosity is a measure of

the internal friction that exists when molecules of lubricant are forced to move

relative to one another.

Most fluids, if the shear rate is not too high, are rheologically classified as Newtonian

That is the shear stress in proportional to the shear rate.

F = A v/h

F/A = force/area = Pascals

= viscosity

h/v = shear rate = meters/velocity = s-1

Therefore, the units of absolute viscosity are: Pa.s or mPa.s this is the viscosity

experienced by all our machines including rolling element bearings


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Kinematic viscosity, the viscosity mentioned on TDSs, is not the absolute

viscosity.

= KV x density if the density of the oil is 1, there is no difference

Between and KV. However, the vast majority of synthetic

hydrocarbon possess a density of approximately 0.82 g/cc

Since PFPEs have a density at 25C of 1.8 g/cc, there is a substantial

difference between the absolute viscosity and the kinematic viscosity

Consider a PFPE fluid with a KV at 40C of 100 cSt. The sample fluid has an

viscosity of 189 mPa.s and thats the viscosity that a bearing wouldexperience.


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Dropping Point ASTM D2265 >260CDensity CTM 0.87 g/cc

Apparent Viscosity

-38C, T-A Spindle 1 RPM

CTM 1 x 107mPa.s

O2Stability, 210C 3500

kPa O2

ASTM D5483 >120 minutes

TGA CTM >313C

Wear ASTM D2266 0.47 mm

Copper Corrosion

24h at 100C

ASTM D4048 1b


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Flash and Pour Point relate to physical properties of oils or the base fluid used

in formulating a grease.

Flash Point is the lowest temperature as which sufficient vapors are liberated by thefluid such that an ignition source will momentarily ignite the vapors.

Pour Point is the temperature at which the viscosity of the fluid becomes sufficiently

high that it will not show movement when held in the large glass test tube horizontally

for 5 seconds. Approximately, 350,000 cSt

Since the flash point of most synthetic oils is greater than 200C, FP is not usually

an issue with rolling element bearing applications.

However, lubricants should be selected for an application with fluidpour points

10 to 15C below the lowest expected service temperature.

Note: The freezing of a lubricant is a physical change that does no permanent

damage.


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Consistency is a measure of how much a grease resists deformation under

stress. This consistency is determined using a penetrometer and for most

bearing applications greases are formulated to an NLGI Grade 2 or Grade 3 consistency

Po is the unworked penetration while P60 is the worked penetration


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Po is the unworked penetration and represents the consistency that rolling

elements experience during the initiation of rolling.

P60 is the worked 60X penetration and represents the consistency of the greaseafter being worked. This is the consistency on which the grade is based.

For example:

NLGI Grade 2 P60 range is 265 to 295

NLGI Grade 3 P60 Range is 230 to 250

Note: Units are 1/10 mm

A Brief Skirmish with Grease Rheology:

Unlike oils, all lubricating greases are rheologically Non-Newtonian


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The rheological duality exemplified by the previous slide demonstrates the advantages

of grease lubricated bearings.

Grease is capable of displaying both solid and liquid-like behavior depending on thethe magnitude of the shear field.

Specifically, when the bearing is at rest, grease acts as a solid and during bearing rotation

grease becomes more liquid-like.

Therefore, when a bearing is at rest, the grease remains in the race regardless of the

bearings orientationdue its solid-like nature at rest.

When the bearing is rotating, the consistency of the grease decreases significantly

reducing viscous drag, heat and energy for rotation.

The amount of solid and liquid behavior exhibited by a grease is determined

using a controlled stress rheometer. G is the storage modulus and is

a measure of the solid component of a grease.


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G is the loss modulus and its value determines the magnitude of the liquid behavior

of the grease.

Since greases have positive values for both G and G, they are often referred to aviscoelastic materials.


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Actual rheogram of grease rheological behavior as a function of shear rate


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As the grease structure degrades, the apparent viscosity of the grease approaches

the viscosity of the base oil.

This rheogram depicts the rheological behavior of a typical oil. Since the shear stress

is a linear function of shear rate, the viscosity is constant.


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Oil Separation is the unwanted loss of base fluid from the grease due to:

Temperature

Pressure

Time

Temperature is a measure of motion and at high enough temperatures the

molecular agitation that exist between the solid thickener and the liquid oil is

great enough to induce oil to escape from the network

For any given temperature, the amount of oil loss is limited since as oil is lost, thethe ratio of thickener to the remaining oil increases making it more difficult

for additional oil loss.

Comment: Typical tests to measure oil loss at 100C are conducted under

static conditions


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Grease containing more oil i.e. NLGI Grade 1, or 0 will inevitably exhibit more

oil loss than firmer greases. Moreover, too much oil loss can be detrimental; but,

some oil separation is necessary for successful lubrication since grease will tend to

remain where it was applied whereas separated oil will migrate to the tribologically

disadvantaged areas.

Under static conditions even mild pressure can induce significant oil separation from

a grease structure.

5% oil loss after 24h at 100C

15% oil loss at RT and 0.25 PSI

I dont have a proven technical explanation why pressure is so detrimental to thestructural integrity of grease other than to suggest that since there is much more oil

in grease than solid thickener, the oil molecules coalesce into mass that can not

be restrained by the neighboring thickener molecules.


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Purchase a 35 pound pail of grease and remove a pound from the center of the

pail and after approximately 3 to 6 months, oil will have collected in the bottom

of the crater. This phenomenon is related to both hydrostatic pressure and time.

This tendency is also related to the base oil viscosity, thickener type and concentration

and ambient temperature.

This oil can be readily returned to the grease structure by simple stirring or if preferred

poured off the grease surface. The consistency of the grease is not expected to change

measurably.

Volatility:

I consider volatility to be a very interesting subject because it encompasses both

the chemistry and physics of lubricant behavior under heat and vacuum.

Lubricant volatility is the loss of all the ingredients in a lubricant due to heat

and, if present, vacuum.


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Since oil is the major ingredient in grease, it is the primary constituent lost

as temperatures rise and atmospheric pressure declines under vacuum.

Definitions:

Volatility = evaporation = the amount of lubricant lost usually, expressed in grams,

as a function of temperature and time.

Vapor pressure is the pressure exerted on the walls of a vessel by those molecules of

lubricant that have entered the gas phase.

The sinister phenomenon:

Outgassing is a sinister phenomenon frequently misunderstood by aerospace engineers,in my opinion.

For simplicity, lets consider a beaker of oil left on a lab bench and undisturbed.

After a given amount of time, the oil will become saturated with air and that air can

be driven from the oil by both heat and vacuum.


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If that beaker of oil is placed in a vacuum oven and the vacuum is pumped down

to a few milliTorr, the air will: expand forming large bubbles, increase in buoyancy,

reach the surface, and explode. Any oil attached to the bubble will be expelled

from the surface.

This also applies to grease.

This is not a structural problem with the lubricant, its an application problem that can

be easily solved by applying gentle heating to the oil to drive the air out of the system

after which the pressure on the oil can be decreased gradually.

Factors that influence volatility and vapor pressure include:

Temperature

Molecular Weight

Molecular Weight Distribution

Time

Surface Area


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VP = 17.14 G ( T/M)

VP = vapor pressure in Torr

G = g/cm2/s

T = absolute temperature in degrees Kelvin K = C + 273.15

M = molecular weight

This is the Langmuir expression describing vapor pressure. If we know the vaporpressure of a lubricant at three temperatures, the lubricants vapor pressure can

be extrapolated to other temperatures if the extrapolation is reasonable

The extrapolation is justified using the Clausius-Clapeyron equation:

Log P2/P1=Hvap (T2-T1) / 2.3 R T2T1 R = 1.987 cal/deg/mole


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Notice that there is no variable relating vacuum to vapor pressure. How can that be

since we all know from experience that lubricants evaporate more readily in space

than they do under terrestrial conditions at the same temperature.

The reason is that a vacuum does directly influence volatility but rather it alters the

equilibrium that exists between molecules evaporating from the surface of the lubricant

and those returning to the surface due to molecular collisions with atmospheric

molecules of gas primarily nitrogen and oxygen.

It may appear inconceivable that a molecule of oil with a molecular weight of 912 g/mole

would reverse its direction after a collision with N2MW = 14 g/mole

Hint: All molecules in the gas phase have the same kinetic energy.


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Grease chemistry:

Multiplyalkylatedcyclopentane thickened with sodium octadecylterephthalamate

Thickener formula: Na C26H42NO3

The grease thickener is formed by the synthesis of NaOH with methyl n-octadecyltere-

phthalamate:

NaOH + CH3-(C=O)- -(C=O)-NH-(CH2)17-CH3

Na+-O-(C=O)- - (C=O)-NH-(CH2)17-CH3 + CH3OH


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This is the fundamental chemistry behind the preparation of Pennzane based grease.

The base oil used in formulating this grease has a very impressive vapor pressure for

an oil with a molecular weight of 912 g/mole.

Post manufacture processing may include homogenization, brought into grade with

additional oil, ultrafiltration and QC testing.

Ultrafiltration removes most particulate contamination from the grease and also removes

potentially troublesome agglomerates of thickener.

In my opinion, any grease intended for a precision bearing applications, should be

ultrafiltered.

Ultrafiltration is defined as no more than 1000 particles/cc below 35 microns

No Particles 35 microns or larger along any axis.


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Note: Ultrafiltration does not remove thickener, additives or oil molecules.

Greases are typically filtered through 10 micron filters and oils through 0.45 micron filters.

In fact, based on the physical evidence, UF improves the tribological behavior of grease since

the consistency becomes firmer and from a purely theoretical standpoint I suspect there is

greater homogeneity between the liquid and solid phases.

Improved homogeneity should result in grease being less likely to dam the inlet to the contactversus unfiltered grease.

However, the most important benefit of ultrafiltration is removing particulate matter from

lubricants that may include metals, glass, dirt, and carbon. All potential stress risers!

Note: Ultrafiltered grease is only an option if it is used in a manner that takes advantageof its cleanliness.

UF grease should only be purchased in plastic syringes, cartridges, or plastic jarsnever

in paint cans.


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Lubricants intended for the long term lubrication of REBs must be fortified

with additives that react chemically with the metal surface during incidental asperity

contact. These additives are usually phosphorus based esters.

I refer to the additives as boundary additives or anti-wear additives. They are not EP

agents. The instrument used to measure antiwear behavior is a 4-ball tribometer.


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0

0.02

0.04

0.06

0.08

0.1

0.12

0 10 20 30 40 50 60

CO

F

Minutes

COF versus Minutes

Under boundary conditions, the coefficient of friction is approximately 0.1


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Ball X1 X2 X3

mm 0.4701 0.4828 0.4445

Average 0.4658 mm s 0.019mm


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Test conditions are typically:

Load = 40 Kg

Temperature = 75C

Duration = 1 hour

Speed = 1200 RPM

Metallurgy = 52100

Lower three test specimens are held stationary while the upper ball is rotated

These always seems to be some asymmetric wear which I attribute to fixturingproblems.

Wear scars are measured in our laboratory at 20X and captured with a digital

camera.


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The large bright spot is NOT the wear scar but reflection from the light source.

Another important lubricant laboratory instrument is a pressure differential scanning

calorimeter. A PDSC is used to rapidly assess the thermooxidative stability of

both oils and greases.


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In this test the thermooxidative stability of a lubricant is determined by heating

milligrams of sample under an atmosphere of pure, dry oxygen at some

fixed temperature. Typically, lubricants are tested at 210C as a function of time

until the samples oxidizes liberating heat.

Oxidation represents a significant exothermic reaction and this chemical event

is recorded by the calorimeter and stored by the software for subsequent

analysis.

By measure the oxidation induction time at three temperatures, the collected datacan be used to extrapolate the life expectancy of the lubricant at other temperatures.

Lets consider an example with a synthetic ester fluid formulated for a sintered metal

bearing application.


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C K 1/K OIT Log OIT mg

250 523 0.00191 8.99 0.95 1.0

225 498 0.00200 58.29 1.76 3.6

200 498 0.00211 306.4 2.49 5.4

y = 7661.1x - 13.64

R = 0.9923

0

0.5

1

1.5

2

2.53

0.0019 0.00195 0.002 0.00205 0.0021 0.00215

LogOIT

1/K

Ester Fluid


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Whats the shelf life of the ester lubricant at 25C?

Based on the linear regression equation, we have: Y = 7661X13.64

25C = 273.15 + 25 = 298K

Log Y = 7661(1/298)13.64

= 12.0

Y = 1012 = 1.2 E12 minutes

=1.2 E12min x 1h/60min x 1d/24h x 1y/365d

= 2.2 million years!

What now? How do we convert useless technical information into something we can use?


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What if we ask what is the highest exposure temperature the lubricant could

tolerate and have a 10 year life expectancy based on the data?

Rearranging the equation in terms of X, we write:

X = (Log Y +13.64) / 7661

10y = 10y x 365d/y x 24h/d x 60min/h =5260000 min.

X = (Log 5.26E6 +13.64) / 7661

= 0.0027

X = 1/K = 1/0.0027 = 376K

C = 376 -273 = 103C Now thats more useful information.

We can expect this ester fluid to last 10 years at 100C if the only mode of degradation is


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thermooxidation.

Important point to remember: The PDSC only requires milligrams of lubricant for testing. If

the OIT of the lubricants is benchmarked when a bearing is lubricated, the condition of thelubricant can be monitored as a function of running time

All greases contain a solid thickening agent that can be utilized to study the condition of the

grease if the thickener has a melting point.

In the realm of thermodynamics, an ice cube and an iceberg have the same melting point

i.e. 0C

However, which piece of ice requires more energy to melt?

Although the answer is obvious, the technical nuance is that if we know how much

energy is absorbed to melt something, we can determine how much of the something:there is!


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Consider a PFPE grease thickened with PTFE, a highly crystalline fluoropolymer with

A melting point of approximately 320C.

The fusion of a solid polymer like PTFE results from the absorption of thermal energy.

The energy absorbed, or enthalpy of fusion, is expressed as:

H = mCpT

Where m = mass of the PTFE

Cp is its heat capacity

T = the temperature change


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We can assume that Cp and the temperature change during melting are constants.


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% PTFE Joules/g

100 59.21

40 23.01

30 17.16

27.5 13.11

20 10.90

10 5.60

Enthalpy of Fusion versus % PTFE


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PTFE in PFPE Grease

y = 1.65x + 2.4448

R2= 0.997

0

20

40

60

80

100

120

0 10 20 30 40 50 60 70

Joules per Gram

%PTFE

Now we have a statistically sound equation that can be used to determine

the thickener to oil ratio in PFPE type greases.


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PFPE grease are frequently used in numerous aerospace bearing applications

and these grease are particularly prone to dramatic consistency changes

when a minor amount of oil is lost from the grease structure.

The grease advantage of this technique is that only milligrams of grease are

needed for the analysis.

This presentation has been more eclectic than I first anticipated, but it does give you

an idea as to why my golf game is so bad.

Thank you for attending this course and I would welcome any questions.

9 - Paul B AMS_Presentation_1!4!2012

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