bearing clearances in relation to oil vis.

[James408]

Hey guys it me, the one who has been beating to death the topic of 15w40 or 20w50.

Ok, my questions is

Does anybody have a chart or info on the relation of main and rod bearing clearances to viscosity recommendations? For example: .002 clearance = xWx oil.

 

[moribundman]

Since Audi recommends viscosities from 5W-30 to 20W-50/10W-60, I don't buy the argument that viscosity the viscosity and bearing clearance are 100% matched. Engine design is a compromise. What's the perfect viscosity for crank bearings may be too thick for rings but too thin for cam lobes, for all I know.

 

[Jimbo]

An automobile where recommended viscosity is critical is the last generation BMW M3. 10W-60 is the only recommended weight due to some rod bearing issue.

Clearances dont tell the whole story. Radial aircraft engines need SAE 60 or 70 (compared to 40 or 50 in other designs) because the loading on the rod journal is so severe and there is not enough room to make it any bigger.

 

[radiometer]

I haven't seen anything to establish that clearances are a factor in viscosity requirements. I have seen what I believe to be credible statements on this forum that the manufacturers that recommend Xw-20 in N.A. call for up to Xw-40 in other countries. CAFE has nothing to do with clearances. Xw-20s might be the wave of the future but I'm personally not yet convinced. However, I'll respect other viewpoints.

 

[1911]

I'll look in one of my Mech E text books at work this week and perhaps give a more comprehensive answer but I can tell you the following off of the tip of stainless 1911 barrel:

If you look at film thickness equations you will not see the bearing clearance mentioned.
You cannot simply lower the clearnce a ton and then use water thin oil and expect to get an oil wedge to form. Each bearing has a characteristic number which is related to the lube viscosity times the rotational speed divided by the radial load. This number relates directly to the tendency or ability for a hydrodynamic film to develop. IF this number is not at a certain value, you can kiss a good stable protective film goodbye.

Thus I believe the low clearance statement justifications for the use of thin oils are utterly ridiculous. A film will not develop unless there is sufficient speed and viscosity whether the clearance is low or high. The hardest lubrication situations are where the load is high and the speed is very low such as for kiln trunnion bearings and the like where the kilns turn at 2 RPM or less. One thing done under such circumstances is to make the bearing much larger than one would expect to lower the radial pressure.

1911

 

[Al]

quote:

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Originally posted by 1911:
If you look at film thickness equations you will not see the bearing clearance mentioned.

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Exactly; it a matter of bearing design. The design will be based on speed and the nature of the load. The clearances, materials, and area will be a part of the bearing spec. The lubricant will be a part of the specfication for the bearing.

There are also generic bearing charts for both anti-friction bearings and plain bearings which will give a lubricant based on the linear velocity of the journal. But again clearance is not part of that equation.

 

[James408]

Ok, 1911 and Al, correct me if am wrong. A tighter bearing clearance does not neccessarily mean a recommendation of thin oil.

 

[1911]

quote:

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Originally posted by James408:
Ok, 1911 and Al, correct me if am wrong. A tighter bearing clearance does not neccessarily mean a recommendation of thin oil.

---

Thanks James,

As Fonzi would say, that's an "Exactamundo". Another even crazier piece of BITOG nonsense is "they recommend thin oil because the oil lines are smaller". This one is really funny. Again, you need a certain amount of lubricant viscosity at the rod and main bearings to get a good hydrodynamic film and this fact is quite independent of oil line diameters!! The rod bearings typically have about 3 times the radial load as the mains. At a given amount of flow volume, making the oil line diameters very thin causes a flow restriction which implies higher frictional pumping losses. Of course, a higher overall flow velocity is also needed to maintain the same flow volume. If these passages were way wider than they needed to be or if more room is needed, then some designers may have sized them down somewhat and having a thinner lube will compensate for some of the extra frictional losses in pumping through thinner lines. However, what does this have to do whether or not a thin lubricant is acceptable at the journal bearings and cam lobes?

The bottom line is that any designer will tell you that the selected viscosities are based upon what is necessary to lubricate the bearings!!!! Thus, the oil lines /oil pump /oil passages must be designed based around the flow requirements for the necessary viscosity required by the bearings and not the other way around!!!!!!!! Nobody designs an engine starting out saying, I want hyper thin oil passages and therefore we need a hyper thin lubricant and by the way, I hope the bearing and cams survive the thinner lube!!! The viscosity and flow amounts must match the bearing needs and these requirements are independent of oil line sizes.

1911

 

[XS650]

quote:

---

Originally posted by 1911:
I'll look in one of my Mech E text books at work this week and perhaps give a more comprehensive answer but I can tell you the following off of the tip of stainless 1911 barrel:

If you look at film thickness equations you will not see the bearing clearance mentioned.
You cannot simply lower the clearnce a ton and then use water thin oil and expect to get an oil wedge to form. Each bearing has a characteristic number which is related to the lube viscosity times the rotational speed divided by the radial load. This number relates directly to the tendency or ability for a hydrodynamic film to develop. IF this number is not at a certain value, you can kiss a good stable protective film goodbye.

Thus I believe the low clearance statement justifications for the use of thin oils are utterly ridiculous. A film will not develop unless there is sufficient speed and viscosity whether the clearance is low or high.
1911

---

The bearing characteristic number, or Summerfeld number is defined as:

S = (r/c)^2* other stuff. r is bearing radius and c is bearing radial clearance. Note that r/c is squared. It does matter.

I still pretty much agree with the other things you said.

 

[Hirev]

I think you have to look at the bigger picture not just bearings. Things such a valve train. valve spring pressure, ambient temperature also should be in the picture.

 

[bbcmat]

http://www.chevyhiperformance.com/techarticles/4380/

Ignore the product "pimping" this magazine does - look at the Callies crankshaft charts illustrating flow vs clearance and effects on temperature.

Bearing clearance creates a major "leak" in the oil path - to maintain adequate lubrication after this leak, a certain oil pressure must be maintained (as a function of pumped volume and effective oil viscosity). This is a engine application-specific number.

A camshaft with a lazy lobe will not place the same demands on the oil film as a more aggressive ramp (with matching higher rate springs etc).

The right viscosity at the right pressure for YOUR application is going to come from an experienced source - not a chart about a specific component. Get on the phone with builders that specialize in YOUR type of motor and have "been there done that". One simple sentence like "30 psi idle, 60 psi at 3000 rpm, start at 15W40 and move up from there to maintain those oil pressures" can be worth digging around and asking questions.

Too thin an oil will still wipe out a set of pushrods / rocker arms - even if "normal" oil pressure is maintained with a high volume pump.

It's all about balance - and experience.

MAT

 

[Al]

quote:

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Originally posted by XS650:
S = (r/c)^2* other stuff. r is bearing radius and c is bearing radial clearance. Note that r/c is squared. It does matter.

I still pretty much agree with the other things you said.

---

Good point it does "matter". But (as you are aware

)...there are 3 other factors (Pressure/load, Apparant Viscosity in the bearing, and RPM. This number is then used in graphs/charts to specify other design parameters. And don't forget that this viscosity is not the stated viscosity of the oil at 100C it is inside the bearing. And a viscosity change or a clearance change would change the temperatures which would go back and change the vscosity inside the bearing (yet again). So one can't reallly say that changing clearance is going to affect design/application one way or the other..at least that would be beyond my mentality.
It really is a complex problem.

Thanks for pointing this out. It forced me to get my 1963 Shigley "Mechanical Engineering" book out. Didn't realize how much I have forgotten in these past 37 years.

 

[OMCWankel]

Don't worry Al, you have probably retained more in the past 37 years than the rest of us have learned in the past 40 years

 

[bbcmat]

The mains clearance on my big block Chev crankshaft was based on the expected flexation of a crank of THAT material type with THAT degree of preparation and THAT amount of basic runout under the influence of THAT amount of cylinder pressure in THIS operating rpm with THIS recipricating weight.

Oil VISCOSITY requirements are determined by the required film strength / additive pack for the worst case APPLICATION within the common lubrication path - oil pump VOLUME and PRESSURE are set to accommodate that viscosity.

Want to test theory on your motor - or go with the experience of a trusted builder who has first hand OBSERVATION of failures and successes.

Ask the guy setting valve lash for the upteenth time - looking over at me eating a hotdog in the pits - then running a full second under his best time - electric windows / treaded tires / plates and all.

IMO

MAT

 

[Gary Allan]

Hmmm...does he offer a 7/70 powertrain warranty?

 

[XS650]

quote:

---

Originally posted by Al:
Good point it does "matter". But (as you are aware

)...there are 3 other factors (Pressure/load, Apparant Viscosity in the bearing, and RPM. This number is then used in graphs/charts to specify other design parameters. And don't forget that this viscosity is not the stated viscosity of the oil at 100C it is inside the bearing. And a viscosity change or a clearance change would change the temperatures which would go back and change the vscosity inside the bearing (yet again). So one can't reallly say that changing clearance is going to affect design/application one way or the other..at least that would be beyond my mentality.
It really is a complex problem.

---

Yes, journal bearing design does induce headaches.

 

[MGBV8]

"Note that r/c is squared"

with 3 other factors (Pressure/load, Apparant Viscosity in the bearing, and RPM.

low vis oils for high rev Honda engines may be appropriate, however how does above work in the case changes to low vis oils for other cars.

 

[XS650]

quote:

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Originally posted by bbcmat:
The mains clearance on my big block Chev crankshaft was based on the expected flexation of a crank of THAT material type with THAT degree of preparation and THAT amount of basic runout under the influence of THAT amount of cylinder pressure in THIS operating rpm with THIS recipricating weight.


MAT

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Mat, whoever told you the part about "flexation" was blowing smoke.

The middle bearings are there to keep the crank from flexing. Any crank will flex to take up any realistic amount of clearance in a bearing when the engine is running. Less flex is always better. Crank flex has nothing to do with proper bearing clearance.

 

[427Z06]

Bump. I've must of been on a three day drunk to miss this topic. j/k

Actual bearing engineering discussion and I missed it! Anyway, I'll throw in my two cents:

An acceptable minimum film thickness is also dependent on bearing surface finishes. Finish specification for cranks use to be typically 8 to 12 RA. But when the industry moved to 5W-30 oils, the spec was changed to 6 RA or less. GM calls for 5 RA on many of the engines it produces today. The move to lower viscosity oils coincided with smoother finishes to allow lower minimum film thicknesses.

 

[nickmckinney]

Another trick for bearings are half groove vs full groove, and/or the use of crossdrilled cranks.

Shaft diameter is paramount as well, you can run a much less clearance on a much smaller shaft obviously. Look at the tightness of a piston pin.