Bearing Oil Pressure Trivia

[Bob Bidonde]

How oil pressure develops in a friction bearing such as the babbitt bearings in the connecting rods and crankshaft main bearings of our Model "A" and Model "B" engines is the trivia of this post.

The primary oil pressure in a bearing develops from the forces applied to the oil by the motion of the crankshaft shaft journal (dynamics), and the mechanical loads transmitted by the crankshaft. The pressure developed in the bearing goes into the thousands of PSIG.

Then how does pumped oil get into a bearing if the pump oil pressure is far less than the dynamic pressure in the bearing? The circular motion (dynamics) of the oil in the bearing actually sucks-in oil supplied by the pump and or splash dippers into the bearing. This occurs because the dynamic oil pressure in the bearing has high and low regions, and the low region sucks in pump supplied oil.

Big Point! The oil pump is not the source of the oil pressure in the main and connecting rod bearings. The pump's purpose is provide the bearings with an adequate supply of oil so the bearings do not run dry.

What primary things affect the dynamic oil pressure in a friction bearing? Volume of oil supplied by the pump, oil viscosity, shaft speed (RPM), bearing clearance, bearing roundness, shaft journal roundness and groves in the bearing are the key attributes affecting oil pressure inside of the bearing. Most of these attributes are intuitive, except that the oil groves in a bearing need some discussion.

Crisscrossed oil groves in the babbitt bearings like those in the Model "A" and Model "B" engines have been found to be deleterious to the dynamic oil pressure developed. Think of it this way. The groves disrupt (makes turbulent) the oil flow within the bearing, so the dynamic pressure is less. Note that modern bearings have a straight circumferential oil grove.

All technocrats, sharp shooters, engineers and scientists should refer to the links below for a scientific explanation of how bearing oil pressure develops.

Reference Reading:
http://www.epi-eng.com/piston_engine...e_bearings.htm
http://www.superstreetonline.com/how...s-garage-tech/
http://www.classicinlines.com/OilPressure.asp

[SAJ]

Very nicely stated and illustrated Bob. An interesting facet that has bothered me is feeding low pressure oil (25 to 60 psi say) into the bottom cap of a main bearing where hydrodynamic pressure is in the thousands of psi region-sufficient pressure to generate enough upwards force to resist combustion and inertial loads and ensure the bearing surfaces remain separated by a film of oil.
One might imagine placing a low pressure feed in the cap simply relieves the hydrodynamic pressure wedge and flow takes place out of the bearing rather than into it!
How then do mods like the Piranio centre cap oil feed work? I must assume they do benefit, since many engines use this method.
But I am with George Miller and others in believing a pressure feed should be into the original valve chamber feed tubes and not the caps.
See your other most interesting trivia posts on compression loads and the one on centre cap oil feeding by Flathead.
SAJ in NZ

[PC/SR]

The Pirano center main oiling works by delivering oil to the center main groove, in the same way a modern system delivers oil into the circumferencial groove of the insert. From there the oil is spread across the bearing and picked up by the journal friction in the same way as described in Bidonde's EFI article. It does not make much if any difference whether the oil is delivered into the groove from the top or the bottom, it is still picked up by the rotating journal and even the slight pressure involved probably supplies more oil than gravity. Either way, the wedge breakdown moves across the bearing along the diagonal groove, which is a weakness from the circumferencial groove which only fails in the center. FWIW, I have always found plenty of oil in my center main when it is torn down, and have never had a problem since installing the kit. Attached are a couple of old caps I had laying around, but the right cap is as installed and shows the hole where the tap is made. It is not perfect and if I was at BV with JB turning 6000+ I would certainly get a different system. However, I do hill climb with it up to around 4000.

[SAJ]

Thanks PC/SR,
What concerns me is that a circumferential groove, top-fed, spreads oil under a rapidly increasing pressure into the "squeeze lubrication" area below the journal either side of the groove where high pressures can be maintained by the hydraulic wedge. But a bottom-fed spiral groove provides an upward path in the eccentric area just past the bottom of the bearing where the high pressure can bleed away back up into the oil-well chamfer at the parting line. Instead of being retained at high pressure in the squeeze area until it leaks out the sides of the bearing. This groove oil then feeds down under rotation back into the same area, where high pressures are relieved again ad infinitum.
The direction the spiral groove is formed is most important in ameliorating this bleed effect and I have seen grooves formed the wrong way i.e for an engine that should operate anticlockwise from the front!
It does not sound as efficient at maintaining a high oil pressure over a wide bearing area as a top feed supply.
Quite possibly I am wrong because "a thousand satisfied customers can't be wrong" and I respect your opinion and findings.
And maybe when I worry about interrupted oil films and terminal oil-whip in modern bearings, I am thinking about much higher surface speeds and RPM.
But oil-whip can occur at Bonnevile 6000 rpm speeds, I believe.
Thanks again for these posts.
They really make this forum interesting.
SAJ in NZ
PS I could not see your cap pics while writing this on my phone. In my minds eye I had an incorrect pic of what the grooves do. Maybe it does not matter too much which way the spiral goes.

[SAJ]

PC/SR. I think you were editing your post while I was still replying to your original one. I am at work, being constantly interrupted. You have explained some of my questions now.
SAJ in NZ

[tbirdtbird]

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"How then do mods like the Piranio centre cap oil feed work? I must assume they do benefit, since many engines use this method."

Perhaps not a good assumption. On several levels