Viscosity Shear (Viscosity reduction and permanent shear)

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MolaKule

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quote:

Does this mean that oils formulated with PAO do not suffer permanent viscosity loss as a result of shear rate?

As the mechanical shear pressures bear down under cyclic loads, some molecules (groups of atoms) will shear apart, resulting in a viscosity loss under that load.

Shear stability refers to an oil's ability to resist shear and to regroup if some molecules do shear.

Oils which have a high probability of "regrouping" and resisting molecular shearing are considered "shear stable;" while those suffering permanent shear are not shear stable.

Mineral oils tend to shear with a large percentage of molecules not "regrouping."

The majority of PAO and ester molecules tend to "regroup" after shearing and to resist shear much better, thus are considered more shear stable than mineral oils.

The reason is that synthetic oils have narrow molecular species and sizes that tend to stay together better (better molecular bonding), while mineral oils have a huge mix of molecules of different sizes and boinding forces which may shear permanently.

In formlating a PCMO or HDEO, the larger the molecule the more shear stable it will be, so one starts out with a long-chain carbon base oil (high viscosity) and then thins it down with a lower viscosity base oil to target viscosity.

The theory is that the longer-carbon-chain oil will help reduce shear.

In reality, all oils shear somewhat, but the synthetics shear much less.
 
OK, So the rate of permanent loss of viscosity is simply significantly lower with advanced basetocks. This is improtant to understand but is just one portion of motor oil(and hydraulic&hypoid) performance I am striving to understnad better.

If I am understanding this explanation properly let me pose another question to remove all doubt what I am thinking, please pardon my density on this matter.
Is the rate of shear as measured in the HTHS not always indicative of the long term kinematic viscosity of a lubricant in use?
 
Good question, Bryan.

The Kinematic Viscosity is a measure of the "bulk" viscosity of the oil, whereas the HTHS is a measure of the shearing of the oil under high temps and high shearing loads.

Think of the KV as the average viscosity and the HTHS as the "localized" viscosity under high temps and loads.
 
Thanks once again.
cheers.gif
Your knowledge is priceless.
I now understand HTHS is a temporary state of shear and the bulk oil's KV is more dependent on it's ability to reform after this temporary shearing with some relation to the extent it is sheared in the extreme conditions. I am also taking it that HTHS is important but it is not the end all of performance, rather part of an equation.
 
quote:

Originally posted by MolaKule:

quote:

Does this mean that oils formulated with PAO do not suffer permanent viscosity loss as a result of shear rate?

As the mechanical shear pressures bear down under cyclic loads, some molecules (groups of atoms) will shear apart, resulting in a viscosity loss under that load.

Shear stability refers to an oil's ability to resist shear and to regroup if some molecules do shear.

Oils which have a high probability of "regrouping" and resisting molecular shearing are considered "shear stable;" while those suffering permanent shear are not shear stable.

Mineral oils tend to shear with a large percentage of molecules not "regrouping."

The majority of PAO and ester molecules tend to "regroup" after shearing and to resist shear much better, thus are considered more shear stable than mineral oils.

The reason is that synthetic oils have narrow molecular species and sizes that tend to stay together better (better molecular bonding), while mineral oils have a huge mix of molecules of different sizes and boinding forces which may shear permanently.

In formlating a PCMO or HDEO, the larger the molecule the more shear stable it will be, so one starts out with a long-chain carbon base oil (high viscosity) and then thins it down with a lower viscosity base oil to target viscosity.

The theory is that the longer-carbon-chain oil will help reduce shear.

In reality, all oils shear somewhat, but the synthetics shear much less.


Molakule, this is not correct by any means as longer chain molecules of a given type are inherently easier to permanently shear than smaller ones. Small ones show more volatility and larger ones are less mechanically stable. This is why VII are much easier to shear down than the small molecule basestocks and bigger VII molecules are easier to shear than smaller ones. For example, turbine oils (these lubes will typically contain a Rust and Oxidation package only and no VII) will show less shear stability with higher viscocity basestocks. In fact, it can be hard to find data on this since these lubes are quite shear stable without the big VII molecules in any grade you may buy them and so are motor oils when they do not contain VII. Now let me give you a physical way to see why this is true. By Newton's basic analysis itself, you are surely aware that the velocity of the fluid varies from effectively zero at the non-moving surface (bearing) to essentially the speed of the shaft on the other side of the fluid film. If you have a large molecule that is initially oriented perpendicular to the flow direction, the obvious result is that one end of the molecule needs to be moving a lot faster than the other end and this is not good for the integrity of the molecule. Of couse, in a mechanical manifestation of LeChatelier's principle, the molecule will try to get aligned parallel to the flow to lessen the effect.

Also, we should be referring to HTHS as a high shear rate test, not a high shear load test as you do here even though by I'll cut you a little slack since by the very definition of a Newtonian fluid, the two factors are of course linearly related. In fact, if you were to graph shear stress versus shear rate, you would get a straight line whose slop is the coefficient of viscosity. The linearity suggests a viscosity which is independent of shear rate.

You will always see fluids characterized as to their degree of Newtonian behavior by a plot of apparent viscosty versus shear rate, not apparent viscosity versus shear load and that is why I implore you to refer to the HTHS test in a more scientifically correct manner. Have you studied fluid mechanics?

Also, both PAOs and parafinnic lubes are without VII are close to Newtonian but not perfect. I'll try to find some plots of viscosity versus shear rate that demonstrate this. When you add VII to either, they begin to act in a more non-Newtonian manner that is characterized technically as "pseudoplastic" where the apparent viscosity drops with shear rate. Thus a plot of shear stress versus shear rate takes on root type function (decreasing slope) where the root may be around 0.7 to 0.95 or so. Now remember , the first derivative of this plot (instantaneous slope) is the coefficient of viscosity. It drops as you go to higher shear rates. Other types of materials display the opposite behaviour and these materials are technically referred to as "Dilatant" in behaviour. Clay slurries and sand in water slurries can display this sort of behaviour.

Now 1911 does not know whether PAO molecules of similar size to parafinnic lube molecules are significantly more shear stable if at all. I suspect that the bigger part of the difference in shear stability between mineral lubes and PAO's is due to the increased amount of VII in the latter. I suspect that most on this board suspected that, we don't need to tell them otherwise.

1911
 
quote:

Originally posted by MolaKule:

quote:

You will always see fluids characterized as to their degree of Newtonian behavior by a plot of apparent viscosty versus shear rate, not apparent viscosity versus shear load and that is why I implore you to refer to the HTHS test in a more scientifically correct manner. Have you studied fluid mechanics?

In responding to non-chemists and non-tribiologists, but intelligent people non-the-less, one has to bring the duscussion to the level of the "best analogy" that imparts the basic meaning and this is one reason for that type of response. You're going to lose the vast majority of readers if you explain everything in pure tribochemical terms.

Have I studied Fluid mechanics? Of course, in both undergraduate and graduate school.

A load is a force that is applied to an article. If that article is rotating (as in piston cyclic loads) then there will be a shearing force imparted to the oil.

In measuring the HTHS, the oil is elevated in temperature, and then sheared under constant load.

Higher viscosity PAO's for example, have a greater number of carbon bonds and still show near Newtonian behavior, thus resist shearing.


Brother Molecular Man,

I teach classes in solid mechanics, you surely do not have to talk to me about loads and stresses. I invented them and I invented their definitions and I invented the analysis of them. But you are certainly welcome to ask questions about stress states including stress and strain tensors, the hydrostatic component, the stress deviator component, the three invariant quantities of a stress state, the Von Mises effective stress, the bulk modulus, Lame's constant, Mohr's circle, the nature of dilatation as applied to stress states, the Tresca and Von Mises yield criterion and yield locuses, and the whole rest of the ****ed boat. I will offer free consultation for a quart of GC and a pair of fuzzy dice.

1911
 
Somebody fetch me half a gallon of strong coffee quickly, please.
wink.gif


To quote Austin Powers: "But what does it all mean?" (bickering aside)
 
quote:

Originally posted by moribundman:
Somebody fetch me half a gallon of strong coffee quickly, please.
wink.gif


To quote Austin Powers: "But what does it all mean?" (bickering aside)


dunno.gif
I was lost at "LeChatelier's principle"

Too many variables for my linear mind to f ollow now.
biggthumbcoffe.gif



I do think Molekule is best at breaking these concepts into terms we can understand, but I am interested in learning 1911's position if I can figure out what he's talking about. I have been googling all these terms he is using and they just create a bottomless pit of additional questions.

[ June 08, 2005, 10:09 PM: Message edited by: Bryanccfshr ]
 
It's not really big words, but the unfamiliar, specific, technical jargon, which means little or nothing to anybody but an expert. Imagine if we all were talking in our own field-specific jargon! And imagine what would happen if we were to pretend that expertise in one field necessarily carries over into another field! Not that THAT ever happens. ;-)
 
I do think Molekule is best at breaking these concepts into terms we can understand, but I am interested in learning 1911's position if I can figure out what he's talking about. I have been googling all these terms he is using and they just create a bottomless pit of additional questions. [/QB][/QUOTE]


Hey guys,

I finally found a good website that may help to explain viscosity as related to shear rate with explanatory graphs as I discussed in my post to the Molecular Man. There is also another one about additive types which says "Viscosity modifiers are generally oil-soluble organic polymers. With a given polymer system, shear stability decreases with an increase in molecular weight. The loss due to shear is reflected in a loss in lubricant viscosity. On the other hand, the "thickening power" of the viscosity modifier increases with an increase in molecular weight for a given polymer type. A performance balance must then be established which takes into consideration shear stability and viscosity needs as well as thermal and oxidative stability in actual engine operation. (Lubrizol)"

Now Molakule is a great contributor here, perhaps the best, and probably has forgotten more chemistry than I ever learned. However, his statement about larger molecules having increased mechanical stability was absurd and I feel that the truth of this matter should actually be somewhat intuitive to all and thus should not really require reference.

This website also discusses the sulfonates which are showing up in XOM Clean 7500. All this is at "http://www.oilmedic.com/lubprops.html"

Now to see some of please go to the website of a company which makes viscometers. http://www.brookfieldengineering.com/support/viscosity/index.cfm#4. You will see a basic definition of viscosity which is the shear stress divided by the shear rate that it produces. The shear rate is the velocity gradient within the fluid in the direction perpendicular to the shear stress vector. It is the difference in speed divided by the difference in distance perpendicular to the stress field (assuming a linear velocity gradient). Their graph shows two infinitesimally thick layers of fluid moving at different speeds under the shearing action.

Now with the equation they give there, you can see that the shear stress is equal to the coefficient of viscosity times the shear rate. So if you graph shear stress versus shear rate ,the viscosity is the slope of the curve. Now scroll down a little lower and they show you how the shear stress relates to the shear rate for Newtonian and non-Newtonian fluids. However, these jabronies decided to plot shear rate versus shear stress instead of the other way around such that the inverse of the slope is the viscosity (maybe they think Molakule is hip since that is remniscent of the socialisitc way he chose to look at this?). Please observe the linear Newtonian fluid with constant slope (viscosity). Now, scroll down a bit more and look what happens to the psuedoplastic fluid as the shear rate and shear stress go up. You can see that the shear rate increases more for a given incremental increase in shear stress and the curve gets steeper (remember viscosity is the inverse of this slope). Thus you can see the viscosity dropping with shear rate (or shear stress as Molakule chooses to say). I truly hope the graphs are more clear than my incessant rambling and propensity towards gambling (see below).

I'll be out of town for a few days and I'll try to answer any questions about the little I know when I get back. My expertise is with solids and even for those, it's generally in the static condition (no flow) but the stuff we are discussing here is very fundamental such that someone not that knowledgeable (like Moi) can easily discuss such basic properties.

I'll be going to Vegas on a psychedelic trip
I'm reading murder novels.....trying to stay hip.

1911
 
The Brookfield link is very helpfull, thanks I am taking it to bed with me and will dream of the perfect simplicity of newtonion fluids. Have a good vacation or "trip"
cheers.gif



Edited to add,
The section about Thixotropy and Rheopexy in the Brookfield link seems to be most pertinant to my original querry.

[ June 09, 2005, 12:05 AM: Message edited by: Bryanccfshr ]
 
quote:

You will always see fluids characterized as to their degree of Newtonian behavior by a plot of apparent viscosty versus shear rate, not apparent viscosity versus shear load and that is why I implore you to refer to the HTHS test in a more scientifically correct manner. Have you studied fluid mechanics?

In responding to non-chemists and non-tribiologists, but intelligent people non-the-less, one has to bring the duscussion to the level of the "best analogy" that imparts the basic meaning and this is one reason for that type of response. You're going to lose the vast majority of readers if you explain everything in pure tribochemical terms.

Have I studied Fluid mechanics? Of course, in both undergraduate and graduate school.

A load is a force that is applied to an article. If that article is rotating (as in piston cyclic loads) then there will be a shearing force imparted to the oil.

In measuring the HTHS, the oil is elevated in temperature, and then sheared under constant load.

Higher viscosity PAO's for example, have a greater number of carbon bonds and still show near Newtonian behavior, thus resist shearing.
 
quote:

Originally posted by 1911:
You will see a basic definition of viscosity which is the shear stress divided by the shear rate that it produces. The shear rate is the velocity gradient within the fluid in the direction perpendicular to the shear stress vector. It is the difference in speed divided by the difference in distance perpendicular to the stress field (assuming a linear velocity gradient). Their graph shows two infinitesimally thick layers of fluid moving at different speeds under the shearing action.

1911 [/QB]

OK 1911, after reading the papers and also reading all of your previous post. I see why you are so frustrated with this subject, I also have a grasp on HTHS which is a better measure of effective viscosity when compared to the highly varied SAE ratings. I now realize that it is not a shear rate but a viscosity measurement.

Your explanation went further than I initially needed to go. I was still struggling with newtonion behavior and didn't understand it's relation to effective viscosity when used in terms of HTHS. From what I gather, the more Newtonion the fluids behavior the more accurate the HTHS series is for determining the effective viscosity. While the more non-Newtonion characteristic fluids (those with heavy polymer or VII's) are harder to predict through HTHS viscosity).

Am I straight ?
 
The HT/HS test @ 150C simply tells you how "squeezable" the oil is, which reflects the # of polymeric thickener. All things being equal, the more thickener, the more the oil will shear in service.

The best lab test for actual shear stability in service are the Bosch injector test or the Kurt Orbain tests...

The best way I've found to determine actual shear stability in service is to divide the kinematic viscosity @ 100C, by the HT/HS viscosity @ 150C. The LOWER the result, the more shear stable the fluid is.

For example:

Mobil 1, 0w-40------14.3/3.6 = approx 4.0
S2000, 0w-30--------11.3/3.4 = approx 3.3
Delvac 1, 5w-40-----15.0/4.1 = approx 3.7
Mobil 1, 10w-30-----10.2/3.2 = approx 3.2


So even without using any of these oils, I can tell you with almost absolute certainty that the Mobil 1, 10w-30 and Amsoil Series 2000 will be the most shear stable and the 0w-40 will be the least shear stable. Provided of course, that they are using the same VI modifier....

Tooslick
 
The HT/HS test @ 150C simply tells you how "squeezable" the oil is, which reflects the # of polymeric thickener. All things being equal, the more thickener, the more the oil will shear in service.

The best lab test for actual shear stability in service are the Bosch injector test or the Kurt Orbain tests...

The best way I've found to determine actual shear stability in service is to divide the kinematic viscosity @ 100C, by the HT/HS viscosity @ 150C. The LOWER the result, the more shear stable the fluid is.

For example:

Mobil 1, 0w-40------14.3/3.6 = approx 4.0
S2000, 0w-30--------11.3/3.4 = approx 3.3
Delvac 1, 5w-40-----15.0/4.1 = approx 3.7
Mobil 1, 10w-30-----10.2/3.2 = approx 3.2


So even without using any of these oils, I can tell you with almost absolute certainty that the Mobil 1, 10w-30 and Amsoil Series 2000 will be the most shear stable and the 0w-40 will be the least shear stable. Provided of course, that they are using the same VI modifier....

Tooslick
 
quote:

Originally posted by 1911:
I finally found a good website that may help to explain viscosity as related to shear rate with explanatory graphs as I discussed in my post to the Molecular Man. There is also another one about additive types which says "Viscosity modifiers are generally oil-soluble organic polymers. With a given polymer system, shear stability decreases with an increase in molecular weight. The loss due to shear is reflected in a loss in lubricant viscosity. On the other hand, the "thickening power" of the viscosity modifier increases with an increase in molecular weight for a given polymer type. A performance balance must then be established which takes into consideration shear stability and viscosity needs as well as thermal and oxidative stability in actual engine operation. (Lubrizol)"

Now Molakule is a great contributor here, perhaps the best, and probably has forgotten more chemistry than I ever learned. However, his statement about larger molecules having increased mechanical stability was absurd and I feel that the truth of this matter should actually be somewhat intuitive to all and thus should not really require reference.


I don't have time to read everything but I think I found the problem. You are talking about large molecular weight POLYMER, and Molakule was talking about large molecular weight OIL. You both are correct.
 
quote:

Originally posted by Jason Troxell:

quote:

Originally posted by 1911:
I finally found a good website that may help to explain viscosity as related to shear rate with explanatory graphs as I discussed in my post to the Molecular Man. There is also another one about additive types which says "Viscosity modifiers are generally oil-soluble organic polymers. With a given polymer system, shear stability decreases with an increase in molecular weight. The loss due to shear is reflected in a loss in lubricant viscosity. On the other hand, the "thickening power" of the viscosity modifier increases with an increase in molecular weight for a given polymer type. A performance balance must then be established which takes into consideration shear stability and viscosity needs as well as thermal and oxidative stability in actual engine operation. (Lubrizol)"

Now Molakule is a great contributor here, perhaps the best, and probably has forgotten more chemistry than I ever learned. However, his statement about larger molecules having increased mechanical stability was absurd and I feel that the truth of this matter should actually be somewhat intuitive to all and thus should not really require reference.


I don't have time to read everything but I think I found the problem. You are talking about large molecular weight POLYMER, and Molakule was talking about large molecular weight OIL. You both are correct.


Thanks Jason,

The same rules apply. Why would they not? It's intuitive. Molakule is not right. With a given bond structure, a longer chain is less mechancally stable regardless of size. However, basestocks are such small chains that the difference in mechanical stability between having paraffin chains with 20 carbon atoms versus 26 may not be that big since they are both small compaerd to VII molecules.

Thanks,
1911

However, volatility goes up with decreasing size for sure.
 
Here's one especially for my good buddy, the Doogster, down in Australia...
smile.gif


Amsoil 10w-40/MCF, Motorcycle Oil:

13.9 Cst @ 100C
HT/HS of 4.52 Cp

13.9/4.52 = 3.08

Shear Stability, Kurt Orbahn (ASTM D-6278)
% viscosity change after 120 cycles:

0.495%


FZG (ASTM-D5182) Load Stage Pass
Wear in mm:

Stage 13, 0 mm, ie NO WEAR


If your drinking buddies at ExxonMobil ever sober up and want to benchmark a shear stable 40wt, here it is...
smile.gif


Ted
 
quote:

Originally posted by TooSlick:
The HT/HS test @ 150C simply tells you how "squeezable" the oil is, which reflects the # of polymeric thickener. All things being equal, the more thickener, the more the oil will shear in service.

The best lab test for actual shear stability in service are the Bosch injector test or the Kurt Orbain tests...

The best way I've found to determine actual shear stability in service is to divide the kinematic viscosity @ 100C, by the HT/HS viscosity @ 150C. The LOWER the result, the more shear stable the fluid is.

For example:

Mobil 1, 0w-40------14.3/3.6 = approx 4.0
S2000, 0w-30--------11.3/3.4 = approx 3.3
Delvac 1, 5w-40-----15.0/4.1 = approx 3.7
Mobil 1, 10w-30-----10.2/3.2 = approx 3.2


So even without using any of these oils, I can tell you with almost absolute certainty that the Mobil 1, 10w-30 and Amsoil Series 2000 will be the most shear stable and the 0w-40 will be the least shear stable. Provided of course, that they are using the same VI modifier....

Tooslick


Nifty formula, I applied it to the watery Mobil 1 0w-30 I currently have in use.


Mobil 1, 0w-40------14.3/3.6 = approx 4.0
S2000, 0w-30--------11.3/3.4 = approx 3.3
Delvac 1, 5w-40-----15.0/4.1 = approx 3.7
Mobil 1, 10w-30-----10.2/3.2 = approx 3.2
Mobil 1, 0w-30------10.3/2.99= approx 3.4

I do have a question on where you are sourcing the specs for the M1 10w30 however. On the PDS it states a 10/3.14 ratio(It doesn't matter that much in the scheme of things thoiugh it still works out the same)

When I use up the M-1 0w30 I will either go to the M1 10w30 EP or the Amsoil s3000 in the truck.
 
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