Hydrodynamic lubrication and journal finish

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What part does journal finish play in the creation of hydrodynamic lubrication. Specifically can a bearing surface be too highly polished to drag the oil around and produce the wedge. If a mirror finish is not desirable what would the optimum surface finish be.
 
I never heard of a surface being too highly polished for hydrodynamic lubrication. I believe it can be a factor in low-speed applications, where you sometimes want to create mini reservoirs for oil lubrication. But not for hydrodynamic lubrication.
 
Thanks.

I've never seen a mirror finished crank journal from the factory or re-grinder but someone has mirror polished one by hand and I was wondering if that was a wise thing to do.
 
Originally Posted by Kestas
I never heard of a surface being too highly polished for hydrodynamic lubrication. I believe it can be a factor in low-speed applications, where you sometimes want to create mini reservoirs for oil lubrication. But not for hydrodynamic lubrication.


My thoughts as well, since in fully hydrodynamic lubrication there is total separation of the journal and the bearing due to the oil wedge, which is caused by the relative motion of the journal wrt bearing.

In the cylinder bore you want cross-hatching or laser dot depressions for mini oil reservoirs.

I would think there is point of diminishing returns in terms of journal finish verses oil entrainement.
 
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Originally Posted By: barryh
I've never seen a mirror finished crank journal from the factory or re-grinder but someone has mirror polished one by hand and I was wondering if that was a wise thing to do.

If you are grinding and polishing journals on a ductile iron crankshaft, the direction of finish is critical. If done in the wrong direction, it can wipe out a bearing.
 
Originally Posted By: barryh
Thanks guys. Highly polishing a journal is fine then.


Well, what is the advantage in polishing down to the n-th finish?

Quote:
...The results are reduced parasitic losses, reduced oil foaming and increased oil shedding...


Someone in Marketing has a wild imagination.
shocked2.gif
 
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You can't polish metal to the point that the oil won't "stick" to the surfaces and create the oil wedge.

The polishing reduces the height of the asperities, and means that for a given geometry and urface speed lower viscosity oils (smaller MOFT) can be satisfactory.

Not sure what the slight reduction in MOFT (opening up clearances decreases MOFT) versus the reduction in Asperite height means 'though...is that the realm of diminishing returns.
 
Originally Posted By: barryh
What part does journal finish play in the creation of hydrodynamic lubrication. Specifically can a bearing surface be too highly
polished to drag the oil around and produce the wedge. If a mirror finish is not desirable what would the optimum surface finish be.


To be picky, this is a fairly interesting question, but its in "Technical and White Papers", and there doesn't seem to be one.
 
My thinking is that molecular attraction means no level of polish will reduce drag on the oil below a certain level. Only a different material could do this. Perhaps some surface treatment that makes oil tend to bead up and roll off.

I'm suspicious of the utility of extreme polish levels though because the moment something sticks or there is metal to metal contact, there goes your polish. Furthermore if both surfaces were polished enough it could cause cold welding. After contact the polish level will depend on the bearing material and the surface created by the debris that is generated.
 
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