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MOFT, HTHSV, VII, and wear: It's complicated #5118764 05/28/19 09:21 PM
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Gokhan Offline OP
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There is a school of people on BITOG who think that wear is solely determined by the high-temperature, high-shear viscosity measured at 150 °C and 1,000,000 1/second shear rate (HTHSV) and the larger the HTHSV, the less the wear on the critical engine parts is.

Tribology (friction, lubrication, and wear) is very complicated and I don't like oversimplifying this very complex problem into a single parameter of the oil, namely HTHSV.

Here is an old paper from the dawn of the HTHSV idea, where they measured the minimum oil-film thickness (MOFT) and wear while varying the HTHSV and oil and viscosity-index improver (VII) type and amount. It's a very systematic study. They don't do studies like that anymore.

In conclusion, they determined that the HTHSV does not reliably predict the MOFT and wear. MOFT on the other hand directly correlates with wear and there is a minimum MOFT beyond which catastrophic wear occurs. There is also a minimum HTHSV beyond which catastrophic wear occurs but that depends on the VII type and content (but not on the additive package) and on the engine.

Some of the conclusions:

  • There is a minimum MOFT beyond which catastrophic wear occurs.
  • HTHSV is deficient in determining the MOFT and wear. MOFT can vary by more than a factor of two for different oils with the same HTHSV.
  • Increasing the viscosity beyond the required minimum actually increases the bearing wear.
  • Catastrophic wear only happens in extreme driving conditions such as wide-open throttle (WOT).
  • Only connecting-rod bearings (big-end bearings) experience catastrophic wear, not the main bearings, piston rings/cylinder liners, and the valvetrain, in extreme driving conditions such as WOT.
  • VII type is crucial in determining the MOFT and bearing wear. (I also interpret that as the VII content being crucial.)
  • KV100 is no worse than HTHSV150 in correlating with the MOFT. However, they are both deficient correlators with the MOFT and wear.
  • The data in the paper is suggesting that monogrades usually result in a higher MOFT than multigrades. (My instinct also always tells me to use an oil with the least amount of VII as possible).
  • Applicable shear rates in the bearings are 10,000,000 - 50,000,000 1/second, which is well beyond (10 to 50 times larger than) the 1,000,000 1/second the HTHSV is measured and reported at. HTHSV still decreases within this shear-rate range with the increasing shear rate, which makes HTHSV alone not sufficient to determine the MOFT and bearing wear. (I interpret this as the reason why the base-oil viscosity at 150 C plays a role in addition to HTHSV.)
  • HTHSV is a deficient predictor of an oil's load-carrying capacity and there needs to be a new ASTM Sequence "MOFT test" to characterize a commercial oil's load-carrying capacity before it can be certified. (I find this the most striking! A such ASTM test has never been implemented in the 30 years since this paper was published!)

Enjoy the paper! It's a very good controlled study. The data is exhausting to absorb.

Effect of oil rheology on journal-bearing performance: Part 4 -- Bearing durability and oil-film thickness
Published September 1, 1989 by SAE International in United States
Event: 1989 SAE International Fall Fuels and Lubricants Meeting and Exhibition
T. W. Bates -- Shell Research Ltd.,Thornton Research Centre, Chester, England
G. B. Toft -- Shell Research Ltd.,Thornton Research Centre, Chester, England


Last edited by MolaKule; 05/29/19 10:41 AM.

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Re: MOFT, HTHSV, VII, and wear: It's complicated [Re: Gokhan] #5118786 05/28/19 10:28 PM
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When you plot the data in Table 3 there's more of a trend correlation between KV100, HTHS and MOFT than the summary seems to portray. Especially for testing that was done 30 years ago with whatever testing and data measuring methods they had back then, which would probably look pretty archaic in today's technology to do such studies. Have to wonder how they accurately (and how repeatable were the measurements) they measured down to 1/100th of a micron for the MOFT, and down to 1/100 of a unit on the viscosity numbers 30 years ago. Definitely an interesting test and interesting data, which seems to pretty much follow what most of us here have hashed over a few times.

Click on the attached PDF file for Table 3 data plots, or click on the "Show PDF" button.

Attached PDF document
Oil Property Plots.pdf (69.73 KB, 41 downloads)
Re: MOFT, HTHSV, VII, and wear: It's complicated [Re: Gokhan] #5118791 05/28/19 10:40 PM
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Thanks for posting your summary, interesting information.


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Re: MOFT, HTHSV, VII, and wear: It's complicated [Re: Gokhan] #5118813 05/28/19 11:26 PM
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Thanks for the plots, ZeeOSix! That's very helpful.

There is definitely a correlation between the MOFT and HTHSV. However, as your plot also shows, for some HTHSV values, MOFT can vary by a factor of two, which is too large of a variation to determine the MOFT from the HTHSV alone.

As far as the accuracy of the HTHSV measurements, I don't know, but I'm guessing they did more than one measurement to make sure their results are accurate.

In order to determine the MOFT, they built a capacitor out of the bearing and connecting rod by electrically insulating the bearing from the connecting rod. Then, it's straightforward to determine the MOFT by measuring the capacitance, as the value of the capacitance depends on the MOFT (minimum oil-film thickness) that separates the "plates" of the capacitor, which are the bearing and connecting rod. They can easily measure the MOFT as a function of the crankshaft angle with a one-degree resolution using this simple built-in apparatus. They said the lead wires attached would periodically break in operation but were easy to replace.


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Re: MOFT, HTHSV, VII, and wear: It's complicated [Re: Gokhan] #5118822 05/29/19 12:29 AM
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Originally Posted by Gokhan
Thanks for the plots, ZeeOSix! That's very helpful.

There is definitely a correlation between the MOFT and HTHSV. However, as your plot also shows, for some HTHSV values, MOFT can vary by a factor of two, which is too large of a variation to determine the MOFT from the HTHSV alone.

If you look at your plots, which are very useful, it shows that the highest scatter in MOFT is for the MOFT 3 measurement, which has the highest RPM and lowest MOFT. Since the shear rate is the speed divided by the MOFT, MOFT 3 measurement corresponds to the highest shear rates.

This is suggesting that the simple relation between the MOFT and HTHSV -- the latter measured and reported at 1,000,000 1/second shear rate in a standard fashion -- is failing at higher shear rates, which were reported in the paper to be 10 - 50 times higher than that. To me this is suggesting that different VII types are resulting in different HTHSV values at higher shear rates and the standard HTHSV value reported at 1,000,000 1/second shear rate is not sufficient. Of course, there could be other factors in play as well.

Therefore, I stand by the importance of the base-oil viscosity at 150 C in determining wear, in addition to the HTHSV viscosity. For that reason, for a given HTHSV, I prefer oils with the smallest amount of VII possible if wear is a concern.


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Re: MOFT, HTHSV, VII, and wear: It's complicated [Re: Gokhan] #5118827 05/29/19 12:44 AM
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TLDR!!! LOL


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Re: MOFT, HTHSV, VII, and wear: It's complicated [Re: Gokhan] #5118837 05/29/19 01:32 AM
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I agree that there is more of a correlation between KV100 & HTHS with MOFT than is being made out in the summary.

It really surprises me that in this day and age where more and more cars are having lower viscosity oils specified from the factory that monogrades haven't become more common place. SAE20 should be good for below freezing, how many of us really need anything more? Same goes for SAE16. My Defender which doesn't really get used during the winter could quite easily get away with a SAE30. I've said it before on this forum, I wish I could get Amsoil ACD 10w30 here for a reasonable price. I'd run it in all my vehicles without fail.


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Re: MOFT, HTHSV, VII, and wear: It's complicated [Re: Gokhan] #5118876 05/29/19 04:33 AM
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Table 3 footnotes show that the only difference between the MOFT 2 and MOFT 3 operating conditions was the engine RPM (2500 vs 3000).

One characteristic of journal bearings is the MOFT will increase due to RPM - ie, the hydrodynamic film wedge thickens with increased shaft speed if the viscosity inside the bearing stays constant (which it doesn't).

The oil shearing inside the bearing also increases with increased RPM, which causes more localized heating and viscosity decrease inside the bearing. On a side note, this is why higher viscosity oil is recommended for high RPM track use to help keep the MOFT to an adequate/safe level for bearing protection - you can see that in the plotted data for MOFT 3. In order to increase MOFT to keep the bearings safe you need thicker oil with more HTHSV as the engine RPM and sump temperature increases.

Since the whole trend line of the MOFT 3 data shifted down, then apparently the decrease in the oil viscosity inside the bearing due to shear heating was the larger factor over the film wedge increase due to the RPM increase.

Re: MOFT, HTHSV, VII, and wear: It's complicated [Re: ZeeOSix] #5118904 05/29/19 05:21 AM
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Originally Posted by ZeeOSix
Table 3 footnotes show that the only difference between the MOFT 2 and MOFT 3 operating conditions was the engine RPM (2500 vs 3000).

One characteristic of journal bearings is the MOFT will increase due to RPM - ie, the hydrodynamic film wedge thickens with increased shaft speed if the viscosity inside the bearing stays constant (which it doesn't).

The oil shearing inside the bearing also increases with increased RPM, which causes more localized heating and viscosity decrease inside the bearing. On a side note, this is why higher viscosity oil is recommended for high RPM track use to help keep the MOFT to an adequate/safe level for bearing protection - you can see that in the plotted data for MOFT 3. In order to increase MOFT to keep the bearings safe you need thicker oil with more HTHSV as the engine RPM and sump temperature increases.

Since the whole trend line of the MOFT 3 data shifted down, then apparently the decrease in the oil viscosity inside the bearing due to shear heating was the larger factor over the film wedge increase due to the RPM increase.

They claim to have fixed the oil-gallery temperature to 130 °C between MOFT 2 and MOFT 3.

Yes, the torque is the same for these two cycles and you would think the latter would have a larger MOFT.

However, the way they define the MOFT is over a 720° crankshaft angle and you're getting the minimum during the exhaust cycle. (See the figure.) So, I don't know how the RPM is affecting it.


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Re: MOFT, HTHSV, VII, and wear: It's complicated [Re: Gokhan] #5118908 05/29/19 05:25 AM
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Another paper reaching the same conclusion as this one.

Abstract:

Bearing oil film thickness (BOFT) measurements were obtained in the front main bearing of a 3.8-liter engine. Engine speed, engine load, and oil temperature were varied to determine the effect of these parameters on BOFT. For single-grade engine oils, the minimum oil film thickness (MOFT) correlated with the Sommerfeld number (Speed x Viscosity/Load). In addition, it was also determined that the type of dispersant package used in a particular single-grade oil did not affect the oil's MOFT values. For multigrade oils (SAE 5W-30, 10W-30, 10W-40, and 15W-40) MOFT values could not be related to the Sommerfeld number with a high degree of correlation. Although high-shear, high-temperature (HSHT) viscosity was found to be important, the viscosity index (VI) improver type was also a factor in determining MOFT. The contribution of VI improver to MOFT was found to be dependent on the SAE grade and VI improver type.

https://www.jstor.org/stable/44471538

The Bearing Oil Film Thickness of Single and Multi-Grade Oils—Part 1: Experimental Results in a 3.8L Engine
Asoke K. Deysarkar
SAE Transactions
Vol. 97, Section 3: JOURNAL OF FUELS AND LUBRICANTS (1988), pp. 335-348


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Re: MOFT, HTHSV, VII, and wear: It's complicated [Re: Bailes1992] #5118912 05/29/19 05:36 AM
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OEM 0w game = 0w20 = inherently high quality base stocks …
Works in all climates … fixed number in after action analysis …

Re: MOFT, HTHSV, VII, and wear: It's complicated [Re: Gokhan] #5118978 05/29/19 07:26 AM
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Originally Posted by Gokhan
In conclusion, they determined that the HTHSV does not reliably predict the MOFT
And yet their own graph showed a nearly straight line. .... Granted, HTHS doesn't take into account the effects of high polarity surface active polymer esters, moly, tungsten, ZDDP, or any other barriers when the film gets very small. That's where the new 0w20 oils can be used, as they have low viscous drag when hydrodynamic, and the additives needed to form a barrier.



mofthths.JPG
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Re: MOFT, HTHSV, VII, and wear: It's complicated [Re: Gokhan] #5119195 05/29/19 10:40 AM
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Re: MOFT, HTHSV, VII, and wear: It's complicated [Re: Gokhan] #5119252 05/29/19 11:45 AM
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there are real synthetic straight 30's meeting the 10w30 requirement with no viscosity improvers as advertised, of course there are NOT the fake synthetics selling for $5 a qt in 4 or 5 qt containers. this info points to the use of common 15-40 oils for great protection on the cheap unless its very cold temperatures. nice paper by the bye

Re: MOFT, HTHSV, VII, and wear: It's complicated [Re: Gokhan] #5119261 05/29/19 11:59 AM
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Originally Posted by Gokhan

For single-grade engine oils, the minimum oil film thickness (MOFT) correlated with the Sommerfeld number (Speed x Viscosity/Load). In addition, it was also determined that the type of dispersant package used in a particular single-grade oil did not affect the oil's MOFT values. For multigrade oils (SAE 5W-30, 10W-30, 10W-40, and 15W-40) MOFT values could not be related to the Sommerfeld number with a high degree of correlation.


Does this include multi-grade synthetic oils that contain no VII such as 10w-30 PAO/POE oils?


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