Rivnut "spacers"

[Anticept]

Hey all, confronted with an issue that I am having trouble finding a resolution to.

I need to drive a rivnut in composite that is more than just a couple layers. Rivnuts into composite is a tricky business because they could bulge out between the composite layers and cause delamination.

I know spacers exist but I am having a very difficult time finding what they are called and where to buy them. What I am calling a spacer is a metal ring that you insert over the rivnut that has a thickness slightly less than the material you are setting the rivnut into. This allows it to grip, protects the rivnut from damaging the composite, and protects it from corrosion.

I have only seen this in a couple applications before, but this was a long time ago.

I am aware that some practices call for using a washer and that's what I will use if needed, but I wanted to ask about spacers.

 

Thanks!

[GrassStripFlyBoy]

Can imagine what you describe but have never noticed these for sale I can recall.  How about click bond products Click Bond Floating Nutplate Kit | Aircraft Spruce

Pricy and would be located under the thickness of the material, maybe not ideal but a floating nutplate to composite application

[Anticept]

The spacers probably have to be made.

At those prices for that plate, I may just go back in and get my knuckles torn up again to put the nuts back onto the studs. I am remounting the flap relay board (i have the old flap control system) using rubber shock isolators due to now twice it occuring where there are cracking solder joints.

[BravoFoxtrot]

If you can find a thin wall SS tube that has an ID slightly larger than the Rivnut OD, you can cut them to a length about the same as the thickness of the composite. If you have access to the back side you can add a washer there for extra protection of the composite material.

[Jim Meade]

Search "rivet nut composite material" and you'll see several ideas.

[Anticept]

I found a better solution than using rivnuts. I have shock mounts that are studded one side, socketed on the other.

This is just to isolate the vibration on the relay board for the flap motor control. Twice now I had to repair it on mine, and once on another.

[GlennM]

I think they are called backing washers.

[Anticept]

There is indeed a thing called a shoulder washer, no results for backing washer that I can find.

Jim: I spotted a couple rivet nut ideas from that search that I like.

https://www.mcmaster.com/rivets/rivet-nuts-for-plastic-and-composites/

https://www.rivetnutusa.com/plusnut-rivet-nut-rivnut/

Neat ideas for sure, would be a lot simpler than messing with spacers and all the headache that can come with. In the future I'll probably grab some if a job calls for it!

[Roger Lee]

I'd just put a thin washer on the back side. This would allow the Rivnut a good grip over just plastic or carbon fiber. You can also make the washer wider to give it a far better surface area.

[Anticept]

In rare cases, a rivnut can buckle closer to the anvil, and in sheet metal, the metal prevents it from bulging out any further and it will begin buckling in a different place. In composite, it will just de-laminate it. This is especially a problem if the material is relatively thin, but in such a case a washer indeed would be a good solution.

As said, I like those plusnuts, they have a large surface engagement and their design is specifically for more sensitive material. They look very appropriate for the application, and there's no trying to play the game of "hook the washer on the rivnut in a difficult to reach hole!"