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Bearing Film Thickness #530356 08/15/03 09:04 AM
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MolaKule Offline OP
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S.A. Cryoff of Texaco, J.A. Spearot of GM, and T.W. Bates of Shell Research wrote an ASTM Task Force Report describing the measurement of actual oil film thicknesses in engines either motored or in actual operation. (A "motored" engine is one in which an external motor drives the engine under test. This test is most often used to measure friction in an engine or run tests not related to combustion).

1. What two methods are used to measure oil film thicknesses in engine bearings and ring/cylinder surfaces?

2. Any guesses on what they found in terms of film thicknesses with respect to oil viscosity.


Again, searching the various threads will yield most of the information needed to answer the questions.

Re: Bearing Film Thickness #530357 08/16/03 01:35 PM
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Molakule,
Film thicknesses can be measure via electrical means, such as capacitance etc (from reading this site). The capacitance implies a closeness between the components.

I believe that direct measurement is possible with shaft riding probes, or proximity sensors.

Film thickness increases with viscoscity (and rotational speed).

Re: Bearing Film Thickness #530358 08/17/03 11:54 AM
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Didn't those beautiful minded good-will hunters from MIT use a whichimacallit light scope when comparing single grade to multi-grade top ring/cylinder film thickness?
I know where to look, but I'm just too lazy and its late.

Re: Bearing Film Thickness #530359 08/17/03 08:12 PM
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RB Shannon Offline
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1. I believe also they use a piezo type sensor,or at least I seam to recall that from somewhere.

2. Film thickness increases from .75 to 1.5 um. with an increase of a SAE grade,with the increase in viscosity, viscuous drag increase which in turn causes heating and lost HP.

I also read recently in a trade publication that you would have to increase one SAE grade to effectivly overcome a 20*F increase in oil temps.due to a reduction in viscosity due to heating to maintain the same general viscosity.

Re: Bearing Film Thickness #530360 08/20/03 12:31 AM
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MolaKule Offline OP
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I find this topic interesting because it's part of the ongoing debate bewteen viscosity and wear verses EPA requirements.

The two methods used are Resistive and Capacitive. In the first method, the oil film is considered to be a high resistance resistor in a circuit. In the second method (Capacitive), the oil in the clearance is considered as the capacitor's dielectric. The capacitive method is the most used since it has been found to be less variable (more repeatable), and more accurate overall.

Here is the test set-up:

Four different viscosity straightweight (monograde Newtonian) oils of 5, 20, 30, and 40 weights were tested in an operating GM 3.8L V6. A plastic shim was placed on the outside of the bearing shell (for insulation) and an AC signal of greater than 100 kHz and about 500 mV was applied to the journal and the bearing shell. So essentially, the inside of the bearing (the metallic bearing shell), the clearance, and the journal form a capacitor, with the oil film acting as the capacitor's dielectric. The dielectric of the oil is measured and the film thickness is then plotted as a function of capacitance. The measurement is called MOFT, for "Minimum Oil Film Thickness."

Oil inlet temperature to the bearing was maintained at a median temp of 120 C. The engine is brought up to temp for one hour, idled for a few minutes, and then brought up to an rpm of from 1600 to 2500 rpm. The bearing/journal clearance was 20 um and the test started with 40 weight (monograde) oil. The oil film varied from a high of 10 um at a crank angle of 270 degrees to a minimum of 3.8 um at 450 degrees crank angle, all at 1600 rpm.

For every ratchet of viscosity grade downward, the minimum oil film is reduced by an average of 1.25 um.

So by about a 5W, the hydrodynamic film is near rupture at a crank angle of 450 degrees, and the lubrication is now mixed hydrodynamic/boundary. This is where boundary protection additives come into play.

The higher the rpm, the thicker the film for any of the mongrade oils. The highest film thicknesses came at 2500 rpm and 50% torque.

In a another phase of the test, multigrades (Non-newtonian) oils with VII's were used. Overall (a generalization), multiviscosity oils showed slightly greater average film thicknesses, lower bearing temperatures, but higher shear rates than did the monograde's. [to be expected].

From my reading of the four papers on this subject, it appears that the minimum weight of oil that can be used in modern engines is SAE 5 weight, and the maximum weight appears to be 40 weight. I believe this is why 5W30's and 10W30's have been the most popular oils for the last three decades; they appear to to be the median weights for all around service for oil temperatures at approximately 250 F bearing inlet temps. This may be another reason why 5W40 oils are becoming increasingly popular as well.

As an added note, a number of companies were involved in the ASTM task force test:

Exxon/Mobil, Pennzoil, GM, Shell, Texaco, Ethyl, Lubrizol, Rohm and Hass, Southwest research Institute (SWRI), Cannon, and Paramins.

[ August 20, 2003, 03:35 PM: Message edited by: MolaKule ]

Re: Bearing Film Thickness #530361 08/20/03 02:22 AM
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Thanks Molakule.

So from that you can basically say that with a 40 weight oil, particles up to 10um can probably be shed from the bearings without building up on the the entry to the wedge.

Thinner oils need better filters ??

Re: Bearing Film Thickness #530362 08/20/03 10:21 AM
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What kinematic viscosity is SAE 5 weight? The latest J300 SAE chart starts at 20-weight.

I found it interesting that multiviscosity oils showed a slightly greater average film thickness than the mono-grade oils. I would expect them to be slightly thinner.

Re: Bearing Film Thickness #530363 08/20/03 08:35 PM
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Thanks Molakule. [Cool]
Also interested about the multigrade film thickness. Any more info on why that happened? And multi-grade/straight-grade is not a good descriptor any more, IMO. Multigrade seems to be used to imply a content of VII, but that may not necessarily be the case any more. E.g. Red Line 10w30 supposedly has no VII. It certainly does not shear. Would it not then behave like a "straight grade" and be better to refer to it as that? It just happens to have better cold flow than the classic straight grade. You have extolled the virtues of multi-grades many times, but it seems to me they would not apply to Red Line since the term seems to imply content of VII. Yet at the same time you praise Red Line, you reject straight grades. I've been confused by this for a while...

Re: Bearing Film Thickness #530364 08/21/03 12:53 AM
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MolaKule Offline OP
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These terms are synonomous in the technical rags:

Straight-weight = straightgrade = monograde; e.g 30 weight (11.25 cSt) Newtonian Oil that usually has no VII's; suitable for constant load, constant temperature operation. I think monograde is the preferred usage.

Multiviscosity = multigrades; e.g., 10W30, implying a 30 weight oil suitable for cold weather operation and acts as if it has SAE 10 weight viscosity characteristics when cold. Usually infers it has VII's unless it is an ester-based, full synthetic fluid.

An SAE 5 weight is about 2.0-4.0 cSt.

"I would expect them [multiviscosity] to be slightly thinner. "

The VII's molecules in dino or the ester molecules in full synth's, which allow wide viscosity variations, are responsible for keeping the average oil film thicker.

[ August 21, 2003, 04:11 PM: Message edited by: MolaKule ]

Re: Bearing Film Thickness #530365 08/21/03 02:05 AM
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Ok I understand that. I think I was mainly refering to your post about the "damming" affect of monogrades at the ring to cylinder interface, increasing drag.
You said multigrade do not damm up as much and thus have less drag. Obviously this is due to shear of the VII.
Since RL 10w30 would not shear, it would be more like a SAE 30 and thus have more damming. This is what I meant.
You have also mentioned multigrade have better oil control and consumtion. Seems this would not apply to RL either since it would behave more like a monograde.

Re: Bearing Film Thickness #530366 08/21/03 02:13 AM
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MolaKule Offline OP
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If a synthetic oil "behaves" like a multigrade in operation, it would perform the same as a dino multigrade with VII's.

Re: Bearing Film Thickness #530367 08/21/03 02:18 AM
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Alright I'm officially confused! [Embarrassed]

But it is my understanding that it does NOT behave like a multigrade. That is one of the advantages. The only way that it behaves like a multigrade is when you turn the temp down to -25C. AFAIK? I don't see how it can behave like a multigrade if there is no VII. [Confused]

[ August 21, 2003, 05:21 PM: Message edited by: Jason Troxell ]

Re: Bearing Film Thickness #530368 08/21/03 03:06 AM
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Jason;
You are asking the same questions I've been asking.
Can you be newtonian retentive and have your multi-grade too?
Are synthetic engine oils that have no VIIs newtonian in nature?
Nature meaning act like...
Molakule...how would a boundry additive work on a babbit plain braring if the lubricant's film ruptured?
Would the ZDDP plate the lead?
Would borates come to the rescue? Moly?
I think the bearing would smear.

Re: Bearing Film Thickness #530369 08/21/03 03:18 AM
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Don't multi-grades or should I say the new VIIs temporarily shear, then return back to their original form?
That might define multi-grades in operation, not necessarly defined only by their cold flow capabilities and static viscosity numbers taken at various temperatures that qualify those oils as multi-grades.
Jason, it looks like your the only one who read that multi-grade VS mono-grade thread of late.
I think that thread asked more questions than it answered.
MITs good will hunting.

Re: Bearing Film Thickness #530370 08/21/03 03:55 AM
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Perhaps in the future, hydro-dynamic flow in addition to hydro-static drag at varoius temperatures will be the two catagories defining multi-grades.
I'm getting closer ain't I?

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