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Go Back   VAF Forums > RV Firewall Forward Section > Traditional Aircraft Engines
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  #21  
Old 07-01-2022, 12:19 PM
mechmike mechmike is offline
 
Join Date: Feb 2020
Location: Powell River
Posts: 22
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This is what I think:

Quote:
Originally Posted by BillL View Post
.... The thrust surface "could not support hydrodynamic lubrication due to poor geometry from the machined surface".....
A thrust bearing cant work if it's only half way around the piece bearing against it. The oil film wouldn't make the trip around the circle if there's no circle. (

To me the telling part is this photo, there's no good reason that the crank hasn't touched all the way around. Even if you ran it with no oil in it, it would contact the thrust all the way around:
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  #22  
Old 07-01-2022, 01:58 PM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
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Quote:
Originally Posted by mechmike View Post
This is what I think:



A thrust bearing cant work if it's only half way around the piece bearing against it. The oil film wouldn't make the trip around the circle if there's no circle. (

To me the telling part is this photo, there's no good reason that the crank hasn't touched all the way around. Even if you ran it with no oil in it, it would contact the thrust all the way around:
I see gouging marks all the way around. Granted, they are MUCH smaller on one side, but definately there. You can even see pock marks in the flat areas. Pretty easy to put a machinist square in the bore and see if the worn area is perpendicular to the bore. While it would appear from the damage pattern that one side was higher than the other, until you know what was eating it apart, it is hard to say that one side has more damage due to an uneven surface or some other factor. Oil should be pouring out of the bearing clearance, so no need to carry a film around the area; It should be constantly flooded with oil, assuming the correct bearing to journal clearance exists. Have you got a bore gauge inside the bearing to get a clearance measurement? Does your case have alignment dowels. If it does, that makes poor machining less likely, as it would be a royal screw up by the case shop.

I still see concentric grooving patterns which would seem to point to something tearing out those trenches vs two flat surfaces wearing each other down. Then again I don't have a great deal of failure analysis experience, so could be barking up the wrong tree.

Larry
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Last edited by lr172 : 07-01-2022 at 02:09 PM.
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  #23  
Old 07-01-2022, 03:56 PM
bobnoffs bobnoffs is offline
 
Join Date: Jul 2012
Location: n. wi
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what does the rebuilder say?
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dakota hawk/jab 3300 built and flying. sold 6/18.getting serious about the 12. in the hangar now as of 10/15/19
RV-12 kit as of 9/13
RV-12 airworthiness cert. 11/30/2021
first flight 7/25/2022
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  #24  
Old 07-01-2022, 06:11 PM
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BillL BillL is offline
 
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Location: Central IL
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Quote:
Originally Posted by mechmike View Post
This is what I think:



A thrust bearing cant work if it's only half way around the piece bearing against it. The oil film wouldn't make the trip around the circle if there's no circle. (

To me the telling part is this photo, there's no good reason that the crank hasn't touched all the way around. Even if you ran it with no oil in it, it would contact the thrust all the way around:
Mike, valid observation. Hydrodynamic lubrication (oil) requires that the surface finish of the mating parts be sufficiently smooth so that the oil film is greater than the finish, else the tips (asperities) will touch and fail. Interesting that the "untouched" part of the case is not smooth. Now, I don't know what the surface finishes specification of the thrust faces are, but it is very important. This is why cranks are like mirrors. Micron range surface finishes. The untouched case portion does not look shiny.

The finish may be more important than the case offset even if it only carries 1/2 the load as a full circumference.

Do let us know what they say as it appears this engine was somehow not built right. Many possibilities for this, squareness relative to the bore, surface finish of the crank, and case, original axial clearance, and installation of the bearing to provide sufficient oil for this surface. Do check Mahlons comment on lubrication.
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  #25  
Old 07-06-2022, 09:07 AM
mechmike mechmike is offline
 
Join Date: Feb 2020
Location: Powell River
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Engine shop says the crank is still great and they sent some photos.
It's very clear to them that the case halves at the thrusts have not been machined off when the main bearing bore was line bored.

Case is getting sent back to the Case shop that, 30 hrs ago, put the dowels in it, bored it and sold them the certified case.

Case rebuilder will give their failure analysis and course of action once they get their hands back on it.

Mike
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  #26  
Old 07-06-2022, 04:16 PM
Gusmax Gusmax is offline
 
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Location: Sherwood Park, Alberta
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At least there is good news regarding your crank!!
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  #27  
Old 07-07-2022, 01:36 PM
Jim Ball Jim Ball is offline
 
Join Date: Sep 2016
Location: Mobile, Alabama
Posts: 53
Default Continental Prime crankcase thrust surface

Quote:
Originally Posted by lr172 View Post
The crank is supposed to rub against the case in that area. It is what prevents the crankshaft from pulling out of the case. Every engine has thrust surfaces to deal with axial movement of the crank. Most use thrust bearings, but the lyc just rubs the crank against the case.

Larry
Just for general information, Continental's crankcases that are used in Titan engines have steel thrust washers installed. It's one of the improvements that ECi added having been a crankcase repair shop. Continental bought ECi and carries on the product line.
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