9 - paul b ams_presentation_1!4!2012
TRANSCRIPT
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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.