Need recommendations for "Threaded Insert" selection.

[uGlay]

Hello:

I am looking to spec a threaded insert of similar structure to "Heli-Coil" inserts.

-The size is 1/4-20 and would be mated to an aluminum (6061-T6) adaptor plate which will attach an Antenna to an aircraft.
-The inserts may be glued, epoxied or mechanically attached to the adapter plate.
-The antenna has 6 screws.
-I am looking for something NAS/MS compatible and that will not have problems getting FAR certification.

Any advice or general direction is greatly appreciated.

Thanks in advance.

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[bf109g]

uGlay -

You didn't specify whether or not your fuselage was pressurized, so I'm going to assume it isn't.

I'd go with any standard anchor nut (nutplate) like an MS21059. If you have a pressurized fuselage, there are sealed nutplates for that purpose, like an Alcoa F1968.

 

[KENAT]

A floating nut nutplate has tolerance advantages so is well worth considering.

KENAT, probably the least qualified checker you'll ever meet...

 

[der8110]

What you ar looking for is MS21209 which are indeed Heli-Coils. For 1/4-20, what you seek is MS21209C4-10 for .250 long, -15 for .375 long, etc.

These have a locking thread so no worries about the screw backing out. No need for epoxy or Loctite; they naturally stay in place.

I am assuming you are adapting the antenna mounting pattern to some pattern existing on the aircraft. This is what I'd call less than common, but is a well known technque and the advantages are obvious - has value in retrofit or experimental operations. Your adapter would need to be somewhat thick; the -15 (or 1.5x dia) are the minimum needed to develop full screw strength so your adapter needs to be at least .375 thick. Another issue is screws poking through and gouging the skin.

There is also an MS spec for KeenSerts but I'd shy away.

 

[uGlay]

der8110: that is exactly what i was looking for. And regarding the adapter thickness, I do have the space to meet the screw requirements. Thank you very much.

bf109g: the fuselage at that point is not pressurized, however a floating nut plate does not lend itself to this application as my screws are going into an adapter. That adapter will then be secured to the aircraft. Thanks anyhow.

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[49again]

Just curious, der8110, why your "shy away" comment concerning Keenserts?

 

[ewh]

Heli-Coil and Keensert are brand names. The mil specs refer to helical coil inserts and key locked inserts, respectively. It generally doesn't matter what brand they are if they meet the spec requirements. Just one of my pet peeves when checking drawings.

When the people fear their government, there is tyranny; when the government fears the people, there is liberty. - [small]Thomas Jefferson [/small]

 

[uGlay]

Okay another question. I am making the adaptor plate in 3D, and would like to model the helical insert but after checking out my Machinery's Handbook (28th Edition) it turns out that it only shows specs for Metric Helicoils, not for Std.

Anyone know where i can find profile dimensions etc for a 1/4-20 insert?

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[der8110]

1) the driven lock keys create a horrendous stress concentration (sharp corners) - probably not a concern in this application.
2) the overall diameter is greater, less edge distance. Needs more real estate for installation. Probably applies here.
3) locking feature (if any) is typically a nylon patch, not as durable as the hex area in a Heli-coil.

(my apologies for using the brand name, like kleenex and xerox it dilutes the value of the trademark - or gives them instant recognition and therefore market share, depending how you look at it. By all means - use whatever the MS sheet calls it, so you can open source).

The geometry is still 20 threads per inch and the wire is a square profile...top and bottom corners touching...that might be adequate for 3D. Modeling that is going to be the fun job!

Another, maybe obvious point - make sure to do the static/FBD/bolt pattern analysis

 

[49again]

uGlay -
Try MS21209.

der8110 -
OK, that makes sense. If ED or stress is critical, pick helicoils. Although all the locking keenserts I've ever used had a deformed thread area for locking, and helicoils are a bunch cheaper (offset somewhat by a higher installation failure rate - I don't think I've ever seen a ham fisted techie mess up a keensert install....)
Thanks!

Steve R.

 

[tbuelna]

Helicoils suck, MIL-STD versions or commercial versions. All aircraft fasteners should be all metal and self locking, for reasons of reliability. The element that locks the helicoil into the base metal is not very strong. And usually about the second or third time you remove the bolt from the (self-locking type) helicoil, the prevailing torque friction between the bolt and helicoil ID will be enough to back out the helicoil. This is especially prone to happen with smaller thread sizes, since the prevailing torque of the helicoil locking element is very difficult to control with smaller thread diameters.

Keenserts (key-locked inserts) and Rosans (ring-locked inserts) are much more reliable, durable and stronger than helicoils. And if you don't have adequate E/D for Keenserts, something like a Slimsert (swage-locked insert) is still better than a helicoil.

Finally, since this joint is exposed to the weather don't forget to use cres materials for the insert and install the inserts "wet" (ie. with a primer or sealant)into the aluminum base material to prevent galvanic corrosion. And every time you install a bolt into that insert, make sure to check the running torque is within spec. If not, the locking feature is worn and the insert should be replaced.

Good luck.

 

[uGlay]

We have made 3 iterations, using MS21209 "heli-coil", another using MS captured nuts and a third using PEM KFS2-420.

At this point we are leaning towards the 1/4-20 captured nuts mainly because if the installers ruin an insert they would have an easier time putting a new nut into place rather than sourcing the right MS21209 or PEM insert.

Thanks for the suggestions.

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[49again]

uGlay -

What do you mean, MS captured nuts?

And, 1/4-20? I've only been around airplane stuff since the early seventies, but, except for engine hardware, most all fasteners #10 and larger are fine threads...

Steve R.

 

[uGlay]

1/4-28 in the lengths I need are backordered for months, picked 1/4-20 provisionally.

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[Sparweb]

About those "PEM's": that's another brand name. It doesn't have a corresponding MIL spec, AFAIK. I've seen these in some equipment enclosures, and yes, they're handy, but also process dependent. Removing a PEM messes up the original hole, so a replacement job has no strength guarantee. I would also worry about corrosion making the PEM's pull-out strength weaken over time.

If there's any tension on these screws, above what you can apply by hand, I'd avoid a PEM.

If more strength is necessary, how about putting studs into the adapter with loctite? That would be "inside the comfort zone" and much stronger. Fewer bits of hardware rolling around, too.

Steven Fahey, CET

 

[tbuelna]

SparWeb,

Loctite is not typically used in aircraft applications because there is no way to verify that it is providing the necessary locking function other than backing the fastener out with a torque wrench. And of course once you've done that, the loctite bond has been compromised. Self locking fasteners can have their prevailing torque measured at installation, thus providing a measurable check that the self locking feature meets design requirements.

The only acceptable use of thread locking compounds like loctite in aircraft, is when they are applied strictly in accordance with a qualified manufacturing process, under carefully controlled conditions. For a limited number of parts it's much less hassle to QA each installation of a self locking fastener than it is to develop and qualify a process for applying loctite.

Good luck.

 

[Sparweb]

Oh, right, the loctite is more "auto" than "aero". Anyway, installing the MS stud with torque as per SRM or standard practices manual for the aircraft in question would do. Use ASSIST to look up MS9827. Then see if you can find a supplier...
...which may be were the hard part begins. Always fun for us, on a forum, to pull suggestions out of the air - it's something else to get the hardware to arrive at your door.
Good luck!

Steven Fahey, CET

 

[RPstress]

From memory, primary structure fasteners have to be double locked if in a group of four or fewer. I think adhesive thread locking is acceptable as one of the two locking mechanisms if they are double locked. However, it's seven or eight years since I last dealt with these matters, so I may well be out of date.

 

[plasgears]

In defense of Keenserts.

The concept is good if you want a locking female thd. It is based on 'dutchman' pins. The trouble is that the keys are not hardened, so in torque out the keys may shear. I would specify hardened keys to your reqts, and check incoming parts for hardness. Keensert may be available in a round pin design rather than the usual thin flat key.

We used keenserts to avoid lockwire, but we had an issue with key hardness. Go for hardened keys.

Lately, I have been using thd lock, but it may have limitations for high temp.

 

[tbuelna]

RPstress,

You make an excellent point about fault tolerance in a structural fastener installation. To achieve single fault tolerance in a threaded fastener installation, the two locking functions are generally achieved by a) thread friction due to installed preload and b) thread friction due to the deformed thread locking feature.