Main bearing clearances

[CitroenFreak]

Hi,

Does anyone know how oil thinkness affect main bearing clearances. It has been discussed before here thread71-134618, but not is such detail

For example how much clearance must one run if one uses 5W, 10W,.. 30,40, 50 grade oils?

Thanks

Cheers

Ziga

 

[GregLocock]

depends on the crank material, block material, oil minimum temperature, oil maximum temperature, diameter of bearing, length of bearing, oil flow rate, mass of crankshaft, mass of conrod, mass of piston, operating speed of engine, engine layout.

I've probably forgotten something.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

Number of journals.

Balance of oil flow between journals.

Sump volume.

Oil return to sump path and mechanism.

Position of oil supply holes in journal.

Regards

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[CitroenFreak]

Well haven't actually thought about it really, but agree with everything.

Is there a way how to calculate it then?

Should I just start calculating everything like bearing surface, force generated by crank, pistons, condrods at redline on bearing surface,...?

But even then I only have force applied on the bearing surface, I still need to know how much force can a certain oil (synthetic that is) take before it collapses at a certain layer thickness.

Or am I way out?

Cheers

Ziga

 

[patprimmer]

You are making a basic error in your understanding of how the oil functions in the bearing.

It is not the hydraulic pressure due to oil pressure that supports the bearings, it is the film strength of the oil. Film strength is not dependant on oil pressure.

Oil pressure is used to control oil flow through the various bearings. This oil flow cools the bearings.

Correct bearing clearance is necessary to maintain pressure to ensure that the last supplied bearing still gets enough oil flow to avoid overheating.

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[CitroenFreak]

Well, don't know where I have stated that oil pressure created by oil pump supports the bearings?

I only asked what's the clearance I should run at a given oil grade.

Different grade oils run in a differenc temp. zone and can support different pressure loadings, and that's what I'm after.

Cheers

Ziga

 

[BCjohnny]

I'm sure that someone will have a different opinion but....

It's generally accepted that with automotive size crank journals, and in the absence of concrete evidence to the contrary, in an engine intended for performance work, 2 to 2.5 thou on the big ends and 2.5 to 3 thou on the mains is about right. For "normal" use you could close them up half a thou without to many problems. Thrust clearances vary more but around 6 to 8 thou is an accepted figure to shoot for, not being wise to go tighter but I've built a few engines around 12 thou and had no definable problems.

They may seem a little wide, compared to OE, but the increased oil flow through the bearing controls the temps better.

Using lower viscosity oils is no reason to close them up.

John.

"It's not always a case of learning more, but often of forgetting less"