Viscosity what viscosity and how to measure?

[Terry]

From Scott Bairs peer reviewed paper: " A More Complete Description of the Shear Rheology of HTHS Journal Bearing Lubrication. May 2007 TLT magazine." COPYRIGHT STLE

Scott Bair is a STLE member and from Center for HP Rheology, GT, George Woodruff School of Mechanical Engineering.

There has been much discussion here of viscosity and best this and that. Let me share that most here ( even many of the BITOG authors) have no idea what a complex subject this is as relates to Journal bearings in a IC automotive engine.

This article is one the best I have read and am still digesting. It describes what I have been dealing with in testing, R&D and racing engines for years. Our recent work on the RS4 program is defined by many of these formulas and wonderful work Scott has produced. I am just a humble lube testing analyst and appreciate the in depth work of Scott here.

Scott hits a home run by my measure and had support from a grant by Valvoline for the work.

I highly recommend that those who can access this paper and study it.

Basically we know that in the ongoing search for a magic vis for best wear protection and efficiency has been a blind mans bluff for many.

The Abstract states that: " High Pressure viscometers which have been used in the past for rheological properties of EHD can now be used to obtain flow curves for motor oils that extend much deeper in the shear- thinning regime than conventional HTHS viscometers.......Sufficiently accurate theory exists to possibly design lubricants for both high load capacity and low friction by balancing pressure and shear rate - effects."

4 defining viscosity measures for those who dig a little deeper here to ruminate on. From the paper, edited for my typing limits.

"1) Isoviscous. The viscosity independent of pressure and shear rate.
2)Piezoviscous, Newtonian. Viscosity depends of pressure only.
3)Nonpiezo, Shear Thin. Viscosity dependent on shear rate only.
4)Piezoviscous, Shear Thin. Viscosity depending on both pressure and shear rate."

Enjoy, Terry

 

[JAG]

Yeah man! Good to see someone else on BITOG again talking about different types of viscosity. Most people just focus on kinematic viscosity which is at atmospheric pressure and very low shear rate. That's not nearly good enough when analyzing oils in an engine that apply huge pressures and some very high shear rates.

Where can this article be purchased?

 

[Terry]

Jag, as referenced above TLT or Tribology and Lubrication Technology Magazine from STLE. May 2007 edition You might be able to contact Scott Bair at Georgia Tech to get a copy.

One aspect of the work I liked was Scott used a non-Newtonian ref fluid CEC RL174 ( see ASTM D 4741) and what was probably Valvoline 5w30 motor oil.

I heard about this work back when presented at ASME/STLE conference in 2004.

The work attempts (quite successfully IMHO) to identify the ambiguity of correlating experimental film thickness measures with wear and shear thinning.

A key result of the work that I have didn't have the theoretical "mathground" to establish

was that for conventional engine oil base stocks tested to 137 MPa,low shear viscosity be four times greater that at standard atmospheric values at high temps. The definition of piezoviscosity that Bair concludes could be more important than "non-Newtonian effects for bearing operation under severe conditions".

While the actual standard and instruments are being argued about for a possible ASTM level standard test our UOA work both for consumers and high level professional consult has been taking this effect into account for some time. Another neat little reasonable cost capability of UOA PROPERLY interpreted!

Terry

 

[JAG]

I called TLT and they are sending me the May 207 magazine for free. Very cool of them!

I know you read my thread on Traction Coefficients in the PCMO forum but not sure if you read the one on polymeric viscosity index improvers (those evil little devils) or another thread where I posted this:
http://www.mt.luth.se/~rola/tribology/handouts_ehl.pdf

 

[AEHaas]

How about measuring viscosity by just dipping a probe into the oil, no movement required.

http://www.vectron.com/whatsnew/pr041607.htm

It is just something else that shows how complicated oil really is. Maybe the film is thin but deforms less because it resists deformation itself. Is this part of why a thin oil may give the same or better separation force between parts?

Is oil density important for wear resistance?

aehaas

 

[Shannow]

AEHaas,
thanks for the link.

We had a bulk delivery of turbine oil that got sent to the distillate tanks at the power station...very expensive fuel oil it turned out.

I've been sweating over the reverse happening (diesel in my turbine oil).

An in-line viscometer in the line from the make-up tank to the turbine oil tanks will be just the ticket.

 

[pp_rp]

AEHass asked a great question ! And I agree with using a thinner oil.

 

[JAG]

Quote:

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AEHass asked a great question ! And I agree with using a thinner oil.

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He did? You agree with using a thinner oil. Well, I like Cheerios. Clearly the topic of this study is completely misunderstood.

 

[carlprop]

Jag likes Cheerios!

 

[JAG]

I found this article online for free! Finally.
http://findarticles.com/p/articles/mi_qa5322/is_200705/ai_n21287233

 

[AEHaas]

Good find.

It does explain why HTHS measurements as we know them may not correlate well with wear. I have always thought that other factors are involved. Pressure may result in an increase in viscosity whereas HTHS does not account for this. Density has a role for certain but I have not been able to find the related equations that I once read.

aehaas

 

[JAG]

That's right, Dr. Haas. I look forward to future research into this important understanding of viscosity as it truly occurs inside engines. It's a lot different than letting an oil flow down a tube under gravity, but that test is so easy to do. The HTHS test takes it one step further by accounting for temporary shear thinning at (150 C temperature) at high shear rates that does occur in engines. As you said, that test does not subject the oil to high pressures which is a limitation. Accounting for how viscosity varies as a function of pressure and shear rate comes even closer to the complex reality. Different base oils have different pressure-viscosity coefficients. With an understanding of these things, there are opportunities to tweak the formulations in very favorable ways. It's interesting that Valvoline paid for this research.

 

[MolaKule]

Very interesting topic.

It would seem to me that through rheological testing of an oil, one could take the PV coefficients, and using Finite Element Analysis and fluid mechanics, one could analyze the effect(s) of base oils in an engine at various loads, speeds, temperatures, etc.

Now including the effects of additives such as VI polymers really complicates the matter.