Bearing Film Thickness

[MolaKule]

Originally Posted By: used_0il


I think I know now why you said that, for an engine
oil to act like a multi-grade, it must contain VIIs.

Are the following statements true?

The thicker the oil film, the lower the shear rate
and fluid friction.

A mono-grade oil would heat up less than a VII containing
multi-grade oil during it's duty cycle in the bearing space.

VIIs were fully activated at 120C, but lost viscosity due
to temporary shear under stress.

The Illinois Central was purchased by www.CN.ca



Some of the older VII's did shear under mechanical stress, but the newer ones have minimal shear over their lifetimes, and what molecules do shear, recover quickly.

An oil does not necessarily have to have VII's in order to be a multi-grade, since the right mix of base oils can accomplish that.

VII's work at all temps becoming more active as the temp rises.

Off-Topic re: ICRR:

When just out of HS I had my FCC license and worked for a Motorola Service Center doing communications work for business radios, police radar, climbing towers, installing antennas, some broadcast work, etc.

We had a contract with ICRR and one of my responsibilities was to go on the road and track down bad radios along the right-of-way and in locomotives. My territory was W. Mo, Ill., Indiana, Kentucky. One of the most enjoyable jobs I ever had.

 

[Shannow]

Originally Posted By: Shannow
Given that for the mineral oils (and I expect the PAOs), they are Newtonian, their KV150 should be the same as their HTHS.

Multigrades the KV150 is higher then the HTHS, like the KV100 is higher than the HTHS100 (see the supertech VOA thread).

Spectrasyn 4
http://exxonmobilchemical.ides.com/en-US/ds244622/SpectraSyn%E2%84%A2%204.aspx?I=30156&U=0

Has a VI of 126, and an HTHS of 1.46.
A mineral will obviously be a little bit less than that, but the VI of 100 to 126 isn't going to make a wild difference.

As Molakule suggests, the parameter is of limited use in discussion without being a formulated oil


Messed up a little there, in that HTHS is Cp, and KV is Cst, there's a density factor in there.

It's what A_Harman does with his shear stability calcs, calculates a KV150, converts it to Cp, then compares the results from a Newtonian theoretical basis to the realised HTHS.

 

[Shannow]

Originally Posted By: used_0il
"The higher the rpm, the thicker the film from any
of the mono-grade oils".


Went through some of the theory in this thread.

Has the theoretical MOFT versus Somerfeld Number...really the charts are based on Newtonian oils.

Bottom parameter for a fixed geometry is proportional to uN/P

u is viscosity
N is speed (radians per second)
P is applied pressure (load divided by bearing surface area)

more viscosity, more film thickness.
lower load, more film thickness.
More speed, more film thickness.

So in essence, your post is correct, however inertial loads increase exponentially with RPM, and I believe it's the inertial loads that win out in an engine.

Coupled with this is distortion, especially in engines like the flat bottomed 6s of the 60s and 70s. Blocks go walkabout, clearances around the crank get sucked up, and the oil film has to center the crank in the journal as well as resist the load.

Ford specify an ILSAC 30 for their 6 in Oz, but the turbos spec 15W40...3CP versus 4+Cp HTHS.

Originally Posted By: used_0il
I think I know now why you said that, for an engine
oil to act like a multi-grade, it must contain VIIs.


Definition of a multigrade is based on KV100, and a cold temperature performance.

Any oil that meets those two requirements is a "multigrade"...one that has no VII can be labelled as either, or both like the Amsoil ACD...note that it's inherent HTHS is 3.4.

If a genuine multigrade, with VII, it must be advertised with the lowest "W" rating that it meets. I think this was a throwback to the older J300 which allowed 2.9HTHS for 0W, 5W, and 10W 40s, and 3.7 for 15W40 and up.

e.g. http://www.pqiamerica.com/May 2013/lucas.htm

Originally Posted By: used_0il
The thicker the oil film, the lower the shear rate
and fluid friction.


Chicken and egg.

Shear rate is the difference in surface speed divided by the film thickness.

Shear rate and viscosity determine the load carrying ability of the lubricant.

Load carrying ability cycled back through those two determines MOFT.

Look at the above chart, and the line "min f" is the lowest friction line for the bearing geometries shown.

Originally Posted By: used_0il
A mono-grade oil would heat up less than a VII containing multi-grade oil during it's duty cycle in the bearing space.


Absolute extreme conditions, I'd sort of agree.

The top of the bearing, around the feed hole is in the low shear range, the "oil wedge" in the high shear range (HTHS is 10^6 sheaar rate, bearings in engines can be tow or three orders higher.

So a lubricant with say a 2.9 HTHS, which can be achieved with an SAE 20 mono has the same viscosity all through.

An XW30 of 2.9, or even an older XW40 of 2.9 before the 2013 J300 revisions will have considerably higher drag in the low shear regions of the bearing than the SAE20, and make more heat...not much, but a little.

It's one of my criticisms of TGMO...an HTHS of 2.6, but to get the astronomical VI, they make the KV100 higher than it needs, and will waste energy in pumping, flow losses, and in the feed in areas of the bearing.

 

[userfriendly]

Will xW16s address that?
A lower VI may result in a flatter curve
between the extreme temperature spread.

What I mean by that is, SAE 16 will obviously
meet the W numbers with a lower VI than SAE 20.

The two temperatures that give an oil it's VI
is not sympathetic to HTHS, and therefor
misrepresenting the requirement.

 

[userfriendly]

Every time when I go on a density kick I end up on this thread.
''''And skip over or ignore Mola's one line, but I read it this time.

Basically he is saying, correct me if I am wrong;

Ignore the individual components and look at the finished product and how it performs.

To his credit he was the 1st on bitog to connect HTHS to fuel economy, in 2003 by the looks of it.

The only axe I have to grind with high VI lubricants, and it was addressed by Shannow, is their artificially high KV100 values.

That pesky topic appeared for the millionth time recently in the motorcycle section.

We can only go so far with HTHS for fuel economy, but all the rage as of late has been with engine warm-up efficiency and that is where KV100 values have to drop.

If so, how far off the mark can good old SAE 10W be?

 

[Shannow]

Here's a Citgo Data sheet that has HTHS for a newtonian 10W...they have 2.3 listed.

http://www.docs.citgo.com/msds_pi/C10005A.pdf

Some have bleated bitterly that the "VI" calculation pics on thinner oils, but that 10W has a VI of over 120...

Cat T-04 has a minimum HTHS for a 10W (non VIIed as per their preference) of 2.4...New J300 has 2.3 minimum.

So some 10Ws could likely be a "10W16" in the new parlance.

 

[userfriendly]

It looks like the Citgo 10W will pass 5W pumping and MAYBE 0W CCS.
It wouldn't take much tweaking to call it a 5W16.
The KV100 at 41 = the low end of the XW20s.

I couldn't help to notice that Delo 400 SAE 10&20 is very low saps.

What do you think is going on in that camp?

 

[userfriendly]

Maybe building a looser engine might help fuel economy.

An extra .001" bearing clearance would reduce shear rate and increase moft.

 

[Shannow]

Originally Posted By: used_0il
Maybe building a looser engine might help fuel economy.

An extra .001" bearing clearance would reduce shear rate and increase moft.


Increasing bearing clearance all other things being equal reduces the minimum oil film thickness...

First part of the bearing characteristic number is diameter/clearance (squared)...(second part is the X axis of the Stribeck curve).

Increase clearance, reduce MOFT, but you certainly DO reduce friction.

Also increase side leakage, and need more oil volume to keep up and maintain pressure.

The Japanese penchant for reducing viscosity is (IMO, and particularly with Honda offering that they are reducing radial clearance, and increasing bearing projected area....which increases drag) is probably chasing piston friction to the nth degree, and finding that at the current state of play an increase in friction in the bearings still provides a positive trade-off.

 

[userfriendly]

If I was to walk into the pits at a race track and tell the experts there
that increasing clearance reduces minimum oil film thickness in a bearing cavity, they would say "give me some of that stuff your smoking".

Without the graphs and illustrations is there another way of winning that imminent discussion?

What people see is a larger clearance holding more oil, a thicker wedge and therefore a thicker oil film in it's static state.

 

[Shannow]

used_oil,
A poor(ish) analogy is that if the bearing and the shaft were fit for fit, i.e. identical diameters, then any load on the shaft is shared with the same part of the bearing, i.e. all over.

Make the shaft an iota smaller, and it's instantly a line contact, not a common surface area.

So the effective pressure of the load has gone up, and massively.

Snowshoes work better than stillettos.

But it's an oversimplification.

Hydrodynamics requires a gap, that reduces in radial clearance. The oil that attaches to the turning shaft is dragged into the ever diminishing clearance, creates pressure, and pushes the shaft away from the bearing.

The "zero" clearance can't do this.

But the "stilleto" example, the oil is squeezed out the ends of the bearing before it can develop meaningful separation, and doesn't have the area available to act upon.

 

[CT8]

Originally Posted By: used_0il
If I was to walk into the pits at a race track and tell the experts there
that increasing clearance reduces minimum oil film thickness in a bearing cavity, they would say "give me some of that stuff your smoking".

Without the graphs and illustrations is there another way of winning that imminent discussion?

What people see is a larger clearance holding more oil, a thicker wedge and therefore a thicker oil film in it's static state.
Larger clearances need thicker oil to slow the leakage. .Look up the clearances in a latest edition NASCAR engine. clearance article .Those engines are beyond amazing. Different engine uses need different clearances.

 

[MolaKule]

Originally Posted By: CT8
Originally Posted By: used_0il
If I was to walk into the pits at a race track and tell the experts there
that increasing clearance reduces minimum oil film thickness in a bearing cavity, they would say "give me some of that stuff your smoking".

Without the graphs and illustrations is there another way of winning that imminent discussion?

What people see is a larger clearance holding more oil, a thicker wedge and therefore a thicker oil film in it's static state.
Larger clearances need thicker oil to slow the leakage. .Look up the clearances in a latest edition NASCAR engine. clearance article .Those engines are beyond amazing. Different engine uses need different clearances.


Look at this statement from the above link:

Quote:
In a NASCAR engine, rules limit the minimum diameters of the rod and main journals on the crankshaft. The rods are 1.850" in diameter while the mains are 1.999". Most of these engines are running rod and main bearing clearances of .001" or less, and they are doing it with low viscosity racing oils such as 0W5, 0W30 and 0W50. These racing oils are as thin as water and are highly friction modified.


This statement really grates me. No oil is as thin as water. They do have a lot of friction modification.

0W5s' are used for qualifying while 0W30-50's are used for the actual race with 0W30's predomonating. Neither is as thin as water nor do they have the viscosity of water.

 

[userfriendly]

Well you know engine builders, experts on everything.

Highly friction modified, what could that mean?

If that term is being applied to plain bearings, could be; an engine oil that
exhibits a high percentage of temporary viscosity loss.

Enter SAE 0W50, BITOG's "darling oil" of the future.

 

[MolaKule]

Originally Posted By: userfriendly

...Highly friction modified, what could that mean?...


It means that friction reduction chemistry is included in large amounts as part of the final mix.