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07-30-2014, 06:47 AM
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Join Date: Feb 2013
Location: Switzerland, Europe
Posts: 199
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Self locking nutplate question
I have experienced that I will need to bolt and unbolt some assemblies several times during construction, which are fastened by self locking plates.
I am a bit concerned, if the strength of the self locking mechanism will decrease the more often you bolt on and off. I have not found any specs. (e.g. K-1000-3 nutplate) on that.
On one hand it sounds even normal that the nutplates would loose locking strength over time, but if so, then it would rather be impractical for servicing parts during maintenance cycles in the future.
Any builders hints and experiences on that are greatly appreciated. I am sure you VAF guys will know best.
Thank you
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07-30-2014, 06:52 AM
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Join Date: Mar 2005
Location: Dallas area
Posts: 10,769
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Self locking nut plates will hold their locking ability far more than you will wear them.
On the other hand, to prevent the time and trouble of screwing the bolts in and out over the build, I tend to use washer stacks until the final install. By doing this, the bolts don't wear on the locking feature and the bolts are easier to install and remove.
__________________
Mel Asberry, DAR since the last century.
EAA Flight Advisor/Tech Counselor, Friend of the RV-1
Recipient of Tony Bingelis Award and Wright Brothers Master Pilot Award
USAF Vet, High School E-LSA Project Mentor.
RV-6 Flying since 1993 (sold)
<rvmel(at)icloud.com>
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07-30-2014, 07:09 AM
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Join Date: Jul 2009
Location: Mojave
Posts: 4,652
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There is a mil spec for these and IIRC, they are only good for about 8 cycles. The high use units we use on the company jet is about 3 times that, and believe me, we replace a lot of them when the airplanes come back through depot.
For our purposes however, as long as they still meet the run on torque requirement, they are good. For anything #10 and smaller, that means if you can't turn or with your bare fingers, it's good. There are published specs for the larger fasteners out there. Same as a self locking nut.
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WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.
Michael Robinson
______________
Harmon Rocket II -SDS EFI
RV-8 - SDS CPI
1940 Taylorcraft BL-65
1984 L39C
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07-30-2014, 07:53 AM
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Join Date: Dec 2007
Location: Flagstaff, AZ
Posts: 2,653
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I agree with Mel, washer stacks are wonderful. Washers are cheap, buy a couple hundred for AN4 and AN3 bolts and #8 and #6 screws. You'll be glad you did. Also helps with the inevitable "the bolt moved and the washers just rolled under a shelf where I can't find them" syndrome.
As for the number of uses for a nutplate, I could believe the 8-cycle thing but... what is the nutplate being used for? My exterior panels have only been off for inspection and are just now past the cycle limit. But those screws are in tension and their own torque holds them in place - plus easily inspected pre-flight. The screws on the wing tanks have only been off twice since build, so definitely no problem there. The screws inside the cabin I actually ran a tap through because I did not want the locking there; they are still plenty tight and a pain to deal with when I have to remove an interior panel. So much so, in fact, that I replaced the Phillips head screws with hex screws.
My concern would be nutplates used in shear; particularly pivoting assemblies. I can't remember if there is such an application on either of my kits. Maybe the one inside the axle nut that takes the bolt for the wheel pant bracket? But I employ another trick there; a lock washer under the bolt head. Similarly, the plastic washers Van supplied to go under the screws holding the spinner on (boy, those hold surprisingly well). Not useful under countersunk screws, but you can always put a lock washer under flat screws and bolts. And, finally, there is locktite. We are EXPERIMENTAL, so we can use non-standard methods and materials.
But that doesn't mean the question wasn't a good one or that we shouldn't give at least some thought to the proper way to do things. I know a Cessna pilot who keeps a handful of screws handy on pre-flight; he always finds one or two missing on his decades-old aircraft. Me, I'd replace the nutplates if they were that loose (and have, on my RV). But he hasn't lost an inspection panel or fairing in hundreds of hours of flying. Perhaps if he were flying a jet he'd worry more. But I've joked with him about it and he says "Perfect is the enemy of good", meaning if he were to try to fix every small thing, he'd never have the time and money to fly. Just food for thought, not really advice.
__________________
Patrick Kelley - Flagstaff, AZ
RV-6A N156PK - Flying too much to paint
RV-10 14MX(reserved) - Fuselage on gear
http://www.mykitlog.com/flion/
EAA Technical Counselor #5357
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07-30-2014, 08:20 AM
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Join Date: Jun 2009
Location: Danville, IN (West of Indy)
Posts: 199
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Good advice
All good advice. I especially like Mel's washer stack suggestion.
I would also add that MIL spec parts are designed for the harshest of environments and many thousands of cycles. Probably well overdesigned for the life of a typical RV.
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Michael
Danville, IN
RV-7A Canopy/Finishing
N317PU Reserved
2020 Dues paid (February 2020)
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07-30-2014, 08:31 AM
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Join Date: Mar 2005
Location: Dallas area
Posts: 10,769
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Quote:
Originally Posted by flion
My concern would be nutplates used in shear;
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The important thing to remember here is that the nut plate actually holds nothing "in shear". It's the bolt that is in shear. Shear nuts may be smaller and weaker than tension nuts simply because they do not need to have "tension" strength.
__________________
Mel Asberry, DAR since the last century.
EAA Flight Advisor/Tech Counselor, Friend of the RV-1
Recipient of Tony Bingelis Award and Wright Brothers Master Pilot Award
USAF Vet, High School E-LSA Project Mentor.
RV-6 Flying since 1993 (sold)
<rvmel(at)icloud.com>
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07-30-2014, 09:46 AM
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Join Date: Aug 2008
Location: Jacksonville, FL
Posts: 522
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I did exactly what Mel suggests...
Eventually your friends will stop asking why you have all those washers on the bolts and that you should just use shorter bolts   . Do get used to chasing washers around though..
__________________
Wendell VAF#1832
RV-6A 3/4 done...N48JE Reserved 
Build site: www.mykitlog.com/weneng
Donated to VAF in 2020
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07-30-2014, 09:50 AM
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Join Date: Jan 2005
Location: 57AZ - NW Tucson area
Posts: 10,011
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Quote:
Originally Posted by Toobuilder
There is a mil spec for these and IIRC, they are only good for about 8 cycles....
For our purposes however, as long as they still meet the run on torque requirement, they are good. .....
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The prevailing torque is the only requirement I found in several Mil Specs I checked. I could find no use limit numbers.
For those purposes, the prevailing torque requirement is exactly the same as that for the equivalent self-locking nuts.
A Mil-Spec use document said they could replace nuts for all structural applications except for one type - a "single rivet nutplate" - which must be why you don't see those at all... 
__________________
Gil Alexander
EAA Technical Counselor, Airframe Mechanic
Half completed RV-10 QB purchased
RV-6A N61GX - finally flying
Grumman Tiger N12GA - flying
La Cholla Airpark (57AZ) Tucson AZ
Last edited by az_gila : 07-30-2014 at 12:00 PM.
Reason: added "don't" for clarification...:^)
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07-30-2014, 11:07 AM
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Join Date: May 2006
Location: Pocahontas MS
Posts: 3,884
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Quote:
Originally Posted by Toobuilder
There is a mil spec for these and IIRC, they are only good for about 8 cycles. The high use units we use on the company jet is about 3 times that, and believe me, we replace a lot of them when the airplanes come back through depot.
For our purposes however, as long as they still meet the run on torque requirement, they are good. For anything #10 and smaller, that means if you can't turn or with your bare fingers, it's good. There are published specs for the larger fasteners out there. Same as a self locking nut.
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Are you sure you're not thinking about fiber or nylon locknuts, instead of nutplates? Nylon & fiber will wear, but all-metal nutplates just flex to grip the screw/bolt.
The whole point of a nutplate is for areas where there will no longer be access to the back side after assembly, right? I'd hate to think about disassembling a structure after 8 maintenance cycles....
Charlie
(I know fiber-lock nutplates exist, but I haven't seen any except on surplus sites for a loong time.)
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07-30-2014, 12:02 PM
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Join Date: Jan 2005
Posts: 963
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Try Boelube if it's really tight to get the screw into the nutplate.
Bob
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Bob Hassel
NM
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