How to determine strength of potted inserts 2

[Bragi]

I have a problem where I am trying to fasten a class divider curtain on a Boeing aircraft to the top of the overhead bins and extending across the aisle. I am proposing to use aluminum U-channels that will be fastened to the top of the overhead bins using potted inserts (p/n SL3378-3-500) and screws (BACS12FA3K5) to attach the channels to the overhead bin.

What structural analysis do I need to do and how do I estimate the strength. My guess is the determining load wouuld be an in-service load but you would need to determine all other loads.

Can anyone help a first timer with this problem?

 

[rb1957]

"What structural analysis do I need to do ..." not much ?, you're designing a curtain holder. the worst thing that can happen is someone pulling on the curtain, 200 lbs, maybe 300 lbs. by the time you've fastened U-channels, you'll have sufficient strength !


"... and how do I estimate the strength" ... easiest way is to make a sample (of one insert) and test (out-of-plane and in-plane) ... out of plane is probably more critical in this application.

 

[wes616]

Service loads, cabin pressurization.

does Surlock give allowables for the inserts. We usually call out the generic NAS p/n for inserts, so I'm not sure which insert "SL3378-3-500" is.

Our designs generally assume a max load of 300 lbs per insert per 1in panel, probablly slightly more than 1/2 for .5in panel.

Wes C.

 

[SuperStress]

Potted insert strength is much more complicated than is being acknowledged here.

It depends tremendously on potting material, face sheet material and thicknesses, as well as core materials and density.

On top of that, allowables tend to be VERY process-sensitive. Is the core being undercut around the insert? How far?

A one-off pull test does constitute an allowable, at least as far as the FAA is concerned.

Having said all that, I agree with the statement that there isn't a whole lot of load on a curtain rod, as long as you slot one end longitudinally so as not to pick up any induced loads from airplane deflections. Make sure too, that the line-of-action of your load isn't too eccentric, which will cause the channel to try to pry your inserts out. Remember, short couples result in large forces.

If you can get your insert load down to less than 50 lbs. per insert or so, you should be able to substantiate the installation "by observation", as long as your company has a qualified process in place for insert installation.

Hope that helps.

SuperStress

 

[EDCCorp]

That is an interesting question as we just finished the same type of approval on a '57 for the installation of a curtain/curtain rail on top of the overhead baggage bin. At any rate, the inserts should be tested after installation, this is the fastest way to prove their strength. Sometimes the manufacturer has data on the insert strength installed in the same panel with the same adhesive that you will be using, but highly unlikely, so testing for approval purposes is the way to go in this case because it is quick and easy to do a pull test. The loads: aircraft t/o (making sure the curtain rail does not come loose under t/o acceleration), loads due to someone pulling on the curtain (as mentioned previously), and importantly, emergency landing condition loads based on FAR 25 requirements for the aircraft. This comes down to a safety issue, will the curtain come loose in a crash scenario, and if so, will it block the aisle or the exit? Also, if you are looking deeper than just the analysis, cabin safety issues should be addressed: placards to ensure the curtain is fastened open during taxi, take-off and landing are important, aisle width requirements must be maintained, and checking exit sign visibility based on FAR 25. Also just make sure the materials used for the curtain/curtain rail meet the FAR 25.853 requirements. Its' simple to do all of this, just should be addressed. Unfortunately its' only a curtain, it seems like a lot of work for a curtain but it is important from a safety stand point.

 

[eap1782]

There is an ESA publication that covers strength prediction of potted inserts in great detail:

PSS-03-1202 Insert Design Book, Issue 1, June 1987

I used the methods in this book in the past and got great results when compared to experiment (within 5% in all cases)

There are a couple of places you can order the book from, just search in Google in and you will find them.

 

[Bragi]

Of course the OEM views data like this as proprietary data. Typically the panels in the interior Boeing aircraft are made with one inch thich nomex honeycomb core and 2 plies of fiberglass. According to sources I now have the shear strength of a NAS1836-3-11 is 300 lbs. If the panel is made out of one ply the shear strength is 150 lbs. The pullout is 180 lbs for 2 plies and 120 lbs for 1 ply.

If you need the exact details on materials (panel core and plies) contact me directly.

 

[eap1782]

The ESA Insert Design Book is in the public domain and not considered proprietary.