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

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.

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.

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.

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.

flion said:

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.

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..

Toobuilder said:

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...

Toobuilder said:

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.

Click to expand...

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.)

Try Boelube if it's really tight to get the screw into the nutplate.

Bob

bhassel said:

Try Boelube if it's really tight to get the screw into the nutplate.

Bob

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That's a good technique every time for the first use of a screw into a new nutplate.

Washer stacks work and I used that technique sometimes but I also had a supply of 1/4 inch long phillips head #8 screws. I preferred them over the washer stacks.

rzbill said:

Washer stacks work and I used that technique sometimes but I also had a supply of 1/4 inch long phillips head #8 screws. I preferred them over the washer stacks.

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This is okay but you should paint all the heads orange so you don't leave them there by mistake. Otoh a stack of washers is hard to miss.

az_gila said:

The prevailing torque is the only requirement I found in several Mil Specs I checked. I could find no use limit numbers...

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It is a material specification. Just as the surface finish, hole diameter, material, rivet hole dimension, etc.

The product must "meet or exceed" X cycles and retain run on torque....

Similar to the TBO on a Lycoming, if you can get more out of it, that's great!

By accident I found that the nut plate AND the screw wear in with cycles. I noticed that the hardware store stainless #6, #8 screws wear in pretty quickly. Then using a proper aviation screw the install friction torque jumps significantly.

You might experiment with this to get a better feel for it.

Toobuilder said:

It is a material specification. Just as the surface finish, hole diameter, material, rivet hole dimension, etc.

The product must "meet or exceed" X cycles and retain run on torque....

Similar to the TBO on a Lycoming, if you can get more out of it, that's great!

Click to expand...

The older MS21047 specs do not seem to have any usage requirements -

http://www.everyspec.com/MS-Specs/MS2/MS21000-MS21999/download.php?spec=MS21047F.034285.pdf

Did they add them when they became SAE specs?

Or are you referring to later, fancier, more expensive nutplates than we typically use?

The MS21047 -

BobTurner said:

This is okay but you should paint all the heads orange so you don't leave them there by mistake. Otoh a stack of washers is hard to miss.

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Thats true. In my case they were commercial grade. Not sure what finish (possibly black oxide) but they were easily different than cad plated screws.

I've never seen any reference to nutplate replacement after x cycles, in 20 years of both .civ and .mil helicopter maintenance, on multiple models from every "major" helicopter OEM (except the Russian OEMs). Plenty of references to loss of run-on torque, prevailing torque, and locking ability... but never "replace nutplates after x remove/install cycles".

Nutplates are an "on-condition" item, just like common panel hardware.

fiber

I think they are confusing fiber lock nuts with metal nutplates.

Yes, they are "on condition"...

Yes, there is a material specification that states the nut plate must meet or exceed xx cycles. Don't confuse this "minimum specification" with a "recommended replacement schedule". If they meet run on torque, they are serviceable.

...but the point remains they do wear out eventually. We replace them by the thousands in depot.