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  #21  
Old 04-24-2009, 07:53 PM
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GAHco GAHco is offline
 
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Default NAS Realities.

Quote:
Originally Posted by scsmith View Post
Thanks Gil for printing the mil-spec chart for the bolts.

I'm fairly sure that the NAS bolts are 125 ksi, so that would say that they are required to achieve 10,300 lbs. from the chart.

Because of good bolt design, including rolled threads, 2-3 thread run-out at root, and radius under heads, most bolts FAR exceed the specs. So, although the rated strength for design purposes is 10,300, the actual strength is probably much higher. My GUESS is about 14,000 lb. When we get a chance, we'll break one and let you know.
Your basic NAS Bolt is 160 to 180 Ksi Tensile, and a min of 95Ksi Shear.

Reference Book Page,

http://www.gen-aircraft-hardware.com...pdf/nashhb.pdf

If I saw more action with the 12 point nuts I could get the prices down.
They are primarily used on larger aircraft wheels and critical mounts under tension. Most things on RV see little tension, except for the engine mount and propeller attachment.
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Last edited by GAHco : 08-27-2009 at 09:45 PM.
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  #22  
Old 04-24-2009, 08:22 PM
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Default Most Important

What type of primer was used to prevent bolt corrosion and could these bolts not be replaced with Sikaflex?
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  #23  
Old 04-24-2009, 08:42 PM
scsmith scsmith is offline
 
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Default about those U-803's...

Bill Wightman's observations about the saddles are right on. I have actually started machining a re-designed saddle that maintains more than 7/16" thickness across the gear leg, and then has counterbores for the bolt heads so the total height is the same as original. ( I wasn't going to share about those, but it is uncanny how often Bill and I think alike)

Bill's suggestion to heat treat them would increase the strength of the saddle, which would be nice, but not the stiffness. Too bad we can't change the modulus with heat treatment, wouldn't that be cool! The stiffness is relevant because it influences how much flexibility there is in the assembly, and how much preload torque we could put on the bolts before the saddles bend.

Thanks GAHco for setting me straight on the NAS bolt strength. The fact that they are 160ksi+ means they deserve even more preload. Oh, if only Terry were right and we could get a preload to a large fraction of yield, rather than the dismal preload we get now, but the saddles just bend and then start bending bolts, causing non-uniform contact at the nuts, etc.
Thus my (until now secret) beefed up saddles.

The bolt strength shown in GAHco's document further supports that the NAS679 nuts are inadequate based on Mil-5 spec's, since they can't match the 13,150 lb required.(at least under my admittedly less than ideal tensile test conditions)
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Last edited by scsmith : 04-24-2009 at 08:59 PM. Reason: fixed word omission
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  #24  
Old 04-24-2009, 10:45 PM
rvbuilder2002 rvbuilder2002 is offline
 
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Default

This is experimental aviation after all...but since there has been no demonstrated history of the gear leg attachment on an RV-8 to be inadequate, I feel quite strongly that all the increasing and strengthening you are doing will have only one result...

Even worse damage to the fuselage structure should your airplane ever be involved in an incident or accident.

Just one opinion...
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Last edited by rvbuilder2002 : 04-24-2009 at 10:47 PM. Reason: typo
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  #25  
Old 04-25-2009, 03:27 AM
SvingenB SvingenB is offline
 
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This was very interressting, but I am not sure I understand the premises for the discussion (I don't have any drawings). If the bolts are loaded to the point where the nuts start popping off, then clarely the joint needs more bolts or larger diameter bolts. On the other hand, there has been no reports of this happening, so either something else yelds first, or the antissipated forces acting on the bolts are much larger than they really are.

But then again, proper nuts never hurts, unless the real problem here is related to corrosion.
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  #26  
Old 04-25-2009, 07:11 AM
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William Slaughter William Slaughter is offline
 
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For me, the take home message is that I can substitute the 12 point nuts for better wrenching and maintain equal or better strength. Thanks for running these tests!
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  #27  
Old 04-25-2009, 07:37 AM
N62XS N62XS is offline
 
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Default I agree!

Quote:
Originally Posted by RatMan View Post
Very interesting indeed.

Seems you have come up with a solution that has evaded a problem!

Unless I've missed something, I haven't heard of one incident where these nuts have failed. Is this something no one is speaking of?
I've owned an 8, have friends with 8s and followed 8s for years. The only thing on an 8 main gear that needs regular attention is checking the attach bolt torque. I am not aware of any other issue with this area of the airframe, Vans or Grove. Am I too missing something?

Impressive work, but I prefer to explore the shear strength of a good top shelf margarita or a cold premium beer. Southern engineering at its best!

Thanks for the info.
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  #28  
Old 04-25-2009, 08:33 AM
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AlexPeterson AlexPeterson is offline
 
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I say weld in some 1/2" steel plates all across the fuselage and increase the gear leg to 2" thick titanium. Go to 3/4" grade 8 bolts. Put chains on the tires also for good winter traction. Sorry, couldn't help myself!

Second guessing some designers comes with risk - especially this one who has an amazingly good track record. Scott said it well - it may just serve to transfer the failure to a more difficult to repair area. The original analysis of the bolts was good stuff - analyzing the surrounding structure's strength is much more difficult. For example, what happens to the surrounding structure when the bolt(s) in question hit 10,000 pounds?
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  #29  
Old 04-25-2009, 10:13 AM
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DanH DanH is online now
 
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Come on guys....read between the lines.

Steve is a professional aero engineer and no doubt aware of the "weak link" concept, ie simply moving the failure elsewhere. He started the thread with some nut strength numbers. Later he disclosed a review of the clamp saddles. His stated use for his airplane includes rough surfaces. Think about it.

I conclude he has already looked at the gear tower, or intends to do so. I suspect we're not going to hear much about it unless (a) he finds some previously unknown horror story (unlikely), or (b) some easy mod which will improve strength, maybe even prevent tearing out a gear tower in a groundloop.

In the meantime, hard data supports an easy mod as well as a caution. A particular full height nut offers more tensile strength than the stock nut. A particular reduced wrench size nut offers less strength than stock. A particular specialty nut offers more strength and reduced wrench size. Good stuff.
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Last edited by DanH : 04-25-2009 at 10:17 AM.
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  #30  
Old 04-25-2009, 12:14 PM
David-aviator David-aviator is offline
 
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Burt Rutan introduced canard airplane builders to the MS21042 nut (the weakest link in the test) a long time ago because of his fanatic compulsion to keep weight to a minimum in an airplane - if you tossed it up and it came down, it was too heavy, I believe he invented that saying, although I think if you tossed a hand full of nuts up they would all come down at the same time.

The MS21042 nuts do weigh less than any of the other nuts and are quite strong for their size. I believe they came out of NASA space age technology.
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