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Hydrodynamic lubrication and journal finish #4666734
02/15/18 11:42 AM
02/15/18 11:42 AM
Joined: Jan 2016
Posts: 333
Cheshire, England
barryh Offline OP
barryh  Offline OP
Joined: Jan 2016
Posts: 333
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)
Re: Hydrodynamic lubrication and journal finish [Re: barryh] #4666743
02/15/18 11:50 AM
02/15/18 11:50 AM
Joined: Jun 2002
Posts: 12,639
The Motor City
Kestas Offline
Kestas  Offline
Joined: Jun 2002
Posts: 12,639
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.

Re: Hydrodynamic lubrication and journal finish [Re: barryh] #4666751
02/15/18 11:59 AM
02/15/18 11:59 AM
Joined: Jan 2016
Posts: 333
Cheshire, England
barryh Offline OP
barryh  Offline OP
Joined: Jan 2016
Posts: 333
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)
Re: Hydrodynamic lubrication and journal finish [Re: barryh] #4666774
02/15/18 12:33 PM
02/15/18 12:33 PM
Joined: Nov 2006
Posts: 20,562
MA, Mittelfranken.de
Trav Offline
Trav  Offline
Joined: Nov 2006
Posts: 20,562
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/

Last edited by Trav; 02/15/18 12:38 PM.

ASE L1, Master. Deutsch Meisterbrief.
Re: Hydrodynamic lubrication and journal finish [Re: Kestas] #4666929
02/15/18 03:16 PM
02/15/18 03:16 PM
Joined: Jun 2002
Posts: 19,482
Iowegia - USA
MolaKule Offline
MolaKule  Offline
Joined: Jun 2002
Posts: 19,482
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.


Last edited by MolaKule; 02/15/18 03:17 PM.

Why are oil filters placed over chassis rails and cross members?
Re: Hydrodynamic lubrication and journal finish [Re: barryh] #4666979
02/15/18 04:08 PM
02/15/18 04:08 PM
Joined: Jan 2016
Posts: 333
Cheshire, England
barryh Offline OP
barryh  Offline OP
Joined: Jan 2016
Posts: 333
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)
Re: Hydrodynamic lubrication and journal finish [Re: barryh] #4666996
02/15/18 04:29 PM
02/15/18 04:29 PM
Joined: Jun 2002
Posts: 12,639
The Motor City
Kestas Offline
Kestas  Offline
Joined: Jun 2002
Posts: 12,639
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.

Re: Hydrodynamic lubrication and journal finish [Re: barryh] #4667050
02/15/18 05:24 PM
02/15/18 05:24 PM
Joined: Jun 2002
Posts: 19,482
Iowegia - USA
MolaKule Offline
MolaKule  Offline
Joined: Jun 2002
Posts: 19,482
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

Last edited by MolaKule; 02/15/18 05:27 PM.

Why are oil filters placed over chassis rails and cross members?
Re: Hydrodynamic lubrication and journal finish [Re: barryh] #4667198
02/15/18 07:30 PM
02/15/18 07:30 PM
Joined: Oct 2012
Posts: 4,825
Taiwan
Ducked Offline
Ducked  Offline
Joined: Oct 2012
Posts: 4,825
Taiwan
Perhaps some slight increase in startup wear after long stand, if "overpolished"?

Re: Hydrodynamic lubrication and journal finish [Re: barryh] #4668144
02/16/18 06:55 PM
02/16/18 06:55 PM
Joined: Dec 2002
Posts: 41,513
'Stralia
Shannow Offline
Shannow  Offline
Joined: Dec 2002
Posts: 41,513
'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.

Re: Hydrodynamic lubrication and journal finish [Re: barryh] #4708861
03/27/18 06:58 PM
03/27/18 06:58 PM
Joined: Oct 2012
Posts: 4,825
Taiwan
Ducked Offline
Ducked  Offline
Joined: Oct 2012
Posts: 4,825
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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