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#4666734 - 02/15/18 11:42 AM Hydrodynamic lubrication and journal finish
barryh Online   content


Registered: 01/09/16
Posts: 312
Loc: Cheshire, England
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.
_________________________
Barry

1979 BMW R45 (Triple QX 5W40)
2009 Mercedes C180K (Shell Helix Ultra 5W40)

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#4666743 - 02/15/18 11:50 AM Re: Hydrodynamic lubrication and journal finish [Re: barryh]
Kestas Offline



Registered: 06/04/02
Posts: 12342
Loc: The Motor City
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.

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#4666751 - 02/15/18 11:59 AM Re: Hydrodynamic lubrication and journal finish [Re: barryh]
barryh Online   content


Registered: 01/09/16
Posts: 312
Loc: Cheshire, England
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.
_________________________
Barry

1979 BMW R45 (Triple QX 5W40)
2009 Mercedes C180K (Shell Helix Ultra 5W40)

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#4666774 - 02/15/18 12:33 PM Re: Hydrodynamic lubrication and journal finish [Re: barryh]
Trav Offline


Registered: 11/20/06
Posts: 18908
Loc: MA, Mittelfranken.de
Want to see some nice cranks check these out. BTW that's not chrome its micro polished to less than 1 Ra.

http://winbergcrankshafts.com/


Edited by Trav (02/15/18 12:38 PM)
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ASE L1, Master. Deutsch Meisterbrief.

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#4666929 - 02/15/18 03:16 PM Re: Hydrodynamic lubrication and journal finish [Re: Kestas]
MolaKule Offline


Registered: 06/05/02
Posts: 19042
Loc: Iowegia - USA
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.



Edited by MolaKule (02/15/18 03:17 PM)

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#4666979 - 02/15/18 04:08 PM Re: Hydrodynamic lubrication and journal finish [Re: barryh]
barryh Online   content


Registered: 01/09/16
Posts: 312
Loc: Cheshire, England
Thanks guys. Highly polishing a journal is fine then.
_________________________
Barry

1979 BMW R45 (Triple QX 5W40)
2009 Mercedes C180K (Shell Helix Ultra 5W40)

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#4666996 - 02/15/18 04:29 PM Re: Hydrodynamic lubrication and journal finish [Re: barryh]
Kestas Offline



Registered: 06/04/02
Posts: 12342
Loc: The Motor City
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.

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#4667050 - 02/15/18 05:24 PM Re: Hydrodynamic lubrication and journal finish [Re: barryh]
MolaKule Offline


Registered: 06/05/02
Posts: 19042
Loc: Iowegia - USA
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


Edited by MolaKule (02/15/18 05:27 PM)

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#4667198 - 02/15/18 07:30 PM Re: Hydrodynamic lubrication and journal finish [Re: barryh]
Ducked Offline


Registered: 10/25/12
Posts: 4388
Loc: Taiwan
Perhaps some slight increase in startup wear after long stand, if "overpolished"?

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#4668144 - 02/16/18 06:55 PM Re: Hydrodynamic lubrication and journal finish [Re: barryh]
Shannow Offline


Registered: 12/12/02
Posts: 39924
Loc: 'Stralia
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.

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#4708861 - 03/27/18 07:58 PM Re: Hydrodynamic lubrication and journal finish [Re: barryh]
Ducked Offline


Registered: 10/25/12
Posts: 4388
Loc: Taiwan
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.

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