Nylon Insert vs. All Metal Locking Nuts and Nut Plates 1

ISTR a number of discussions on this site about self locking nuts and such; a search should bury you in reading material.

... one such discussion centered on the recent-ish fatal crash at the Reno air races, which included pictures of nutplates with apparently phenolic/cotton or similar old school locking inserts that may have been used for authenticity, but appeared to have been long overdue for replacement.

I think the design selection is based on temperature, but both nylon and metal inserts have a limited number of mate/demate cycles, and I'm not sure where/if/how that's actually specified.



Mike Halloran
Pembroke Pines, FL, USA

The Mil-DTL for patches on screws gave a 5 time use limit, or something about tested for 5 time use or something like that. So suspect that the governing standard for these fasteners may have similar information.

MIL-DTL-25027 for the all-metal.
MIL-DTL-18240 for Nylon.

The prevailing torque requirements are not the same.

The corrosion resistant steel NAS1033 nutplates only come in an all-metal configuration, if that is a consideration for your application.

The nylon locking insert material is probably only good to around -40degF, and exterior locations on many aircraft experience temps below that.

You mentioned the application "....is not critical or structural by any means.", but are there any QA requirements for this application that require verification the correct components were installed?

Nutplates are fairly easy to replace. Drill out two rivets, touch up the surface finish on the part, and rivet the new nutplate in place. So unless there is a large number of nutplates involved, or they are difficult to access, the logical thing to do would be to install the correct nutplates. No paperwork trail for you to create documenting this discrepancy, and your customer gets the quality product they deserve and expect.

Should we also consider the variation in the floating/non-floating features of some nutplates?
IIRC most nylon-locking nutplates are of the fixed variety (sorry don't have NAS1033 here to see your particular type).
The self-aligning and floating types of nutplates offer many advantages that may be important in the choice to keep/replaces these nylon-locking nutplates you have on now.


STF

Some OEM's have a torque value to test nylocs against to determine if they need replacement (Eurocopter AS350 MM comes to mind).

I have always considered nylocs suitable where fasteners are not removed often, and all metal nuts for frequently accessed panels although prefer to use the later. All metal can be harder on the screw finish as well (have come across a rouge batch that striped all the finish off the screws).

SparWeb-

NAS1033 is a right angle floating nutplate. Here is the relevant section of the spec regarding float:

"Float of the nut portion shall not be less than .016 inches in any direction from the centered position. Nut body shall be capable of engagement with a bolt in the maximium misaligned position. Nut shall not float as to exceed dimension B (housing width) or L (housing height). Maximum vertical float within the housing .020 for .1900 size and smaller, .030 for .2500 size and larger."

Thank you tbuelna,
I've recently used a different type (NAS1791) with much more float than that, and chose to use very large clearance holes in order to take advantage of that feature. No, the fasteners were not heavily loaded in that case.

STF

verymadmac,

Good post about when to use all metal vs nylocs.

Can others give advice as to when they use all metal vs nylon insert nuts/nut plates, temperature considerations aside?

Most of my design experience has been in aerospace, both commercial and military, and I don't recall ever using any self-locking nut/nutplate other than all metal types. The program DRD typically included the requirement for using all-metal construction of self-locking nuts/nutplates. I think the primary reason is the belief that all-metal construction provides the best reliability.

I would always use all-metal self-locking nuts/nutplates for any application where safety or reliability might be a concern.

724napier

Non-metallic inserts were generally the 1st generation of thread-locking available... until fastener designers figured-out how to embed locking features in all-metal parts, thereby simplifying, lightening, strengthening and widening the service temperature range of nut/bolt installations where self-locking was required.

The effects of environmental moisture and high and low temperature on the chemical stability and toughness of nylon** can vary dramatically as these [moisture + temperature] conditions interact. Typically nylon [polyamide] has a defined general low-upper temperature limit [~+280F worst environment] and lower temperature limit(s) [highly variable] that is/are dramatically affected by nylon chemistry/type [**such as 12, 6/6, 6/10, filled, unfilled, etc] and chemical/moisture exposure.

On-the-other-hand, all-metal fasteners can have wider upper [+450F] and lower [-100F or lower] temperature service limits, that are essentially unaffected by environmental variations [chemicals, moisture, temperature, etc]; and are genuinely limited by the alloy and/or unique finishes.

To widen the temperature range, reduce environmental effects and increase the number of times non-metallic inserts can be re-threaded [re-used], polyimide plastic inserts have been used in non-standard nuts [and nut-like parts] and in male thread patch inserts... at considerable expense.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

FYI... for giggles...

Following MIL-HDBK has a remarkable amount of info regarding Polyamide [Nylon] Plastics...

MIL-HDBK-797 POLYAMIDE (NYLON) PLASTICS: PROPERTIES, PROCESSING, PERFORMANCE, AND MILITARY APPLICATIONS



Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]