Viscosity spread & VI vs shearing

JAG

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Thank you, Gokhan for correcting my errors. I did that all using my phone and apparently it was beyond my capabilities at the time. I heard that little voice in the back of my head saying that I am making mistakes. As usual, that voice was correct. Yeah, anything over 1 indicates that one or more of the input values has error(s). Well, that is the case as long as the oil is not a shear thickening fluid. I’ve never heard of a base oil or motor oil having a shear thickening property. I’ve only ever heard of Newtonian and shear thinning behavior for such fluids.
 
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basic question if it has not already been answered 40 times crzy can you compare A_Harman index of different weight oil (e.g. 5Wx30 vs. 0Wx20) or would it meaningless like comparing apples and oranges? I am assuming it is not a universal index and it is best to compare similar weight oils! no?
 

JAG

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You can compare across viscosity grades but know that there is a trend of oils with wider spreads of viscosity grades having lower Harman Index. There are exceptions to that trend. It answers the question of how much is its dynamic viscosity reduced at 150 C temperature and 1e6/sec. shear rate, resulting from shear thinning. If it is truly 1.0, it is a Newtonian fluid, by definition. If it’s 0.8, it loses 20% of its dynamic viscosity due to shear thinning at that temperature and shear rate.
 
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Just curious, what is the Harman index for single grade oil for example SAE30. I assume there is no vii in it! Is the index 1 or perfect (if there a such thing) or not applicable? The sae30 still shears. no?
 
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Originally Posted By: OilUzer
basic question if it has not already been answered 40 times crzy can you compare A_Harman index of different weight oil (e.g. 5Wx30 vs. 0Wx20) or would it meaningless like comparing apples and oranges? I am assuming it is not a universal index and it is best to compare similar weight oils! no?
Yup, in fact A_Harman index is often used for comparing different viscosity grades. It measures the temporary shear of the viscosity-index improver (VII) in high-temperature, high-shear (HTHS) conditions. You can loosely relate it to the amount of the VII present. You can then loosely relate that to permanent oil shear. Caveats: (1) The two extrapolations to 150 C done for the viscosity and density can have errors. (2) Different types of VII's shear in different rates. Therefore, if you had, say, 10% VII in two separate oils but they were different types of VII's, you could get different A_Harman indexes for the two oils. Nevertheless, this still makes the A_Harman index useful for getting a sense for the overall effect of the VII's.
 
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Originally Posted By: Gokhan
Yup, in fact A_Harman index is often used for comparing different viscosity grades. It measures the temporary shear of the viscosity-index improver (VII) in high-temperature, high-shear (HTHS) conditions. You can loosely relate it to the amount of the VII present. You can then loosely relate that to permanent oil shear. ...
I assume this answers my other question regarding SAE30 ... Harman index does not apply or used for single grade oil? Also just noticed that A_Harman is posting on this thread re index calculation, etc. Sounds like he came up with this index! Is that true? I did a search and found a racing company ...
 
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Originally Posted By: OilUzer
I assume this answers my other question regarding SAE30 ... Harman index does not apply or used for single grade oil? Also just noticed that A_Harman is posting on this thread re index calculation, etc. Sounds like he came up with this index! Is that true? I did a search and found a racing company ...
Yes, monograde oil doesn't have viscosity-index improver (VII); therefore, it has A_Harman index = 1, assuming a shear-stable base oil. Yes, A_Harman came up with the A_Harman index on the previous pages of this suddenly resurrected thread.
 
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Originally Posted By: OilUzer
Just curious, what is the Harman index for single grade oil for example SAE30. I assume there is no vii in it! Is the index 1 or perfect (if there a such thing) or not applicable? The sae30 still shears. no?
From what we see in the A_Harman index calculations, typical base oils, without VII added, shear negligibly. Therefore, A_Harman index for a typical base oil without VII (a monograde) ~ 1.00. However, base stocks with very large molecules, meaning very heavy base stocks (KV100 ~ 300 - 1000 cSt), shear even worse than common VII polymers. So, yes, base oils shear as well but typically negligibly, as long as they don't use very heavy base stocks. If you use very heavy base stocks, you will shear some of the large molecules into smaller ones, which will decrease the viscosity. For example, after 20 hours, 300 cSt PAO shears almost 20% and 1000 cSt PAO shears almost 50%. 300 cSt mPAO shears only 2%. You can ask where/why they use these heavy base stocks, and they are used in gear oils or as HTHSV/VI boosters in engine oils. My guess for 4 - 8 cSt PAO used in typical base oils for engine oils, shear is a fraction of a percent but I don't have actual data.
 
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