Rail Button Question

[KenECoyote]

Has anyone tried rivet nuts? It would be hella strong, but I don't know if it would damage the fiberglass or cardboard. I may do some testing with ones I have from my custom car work.


Actually, I used them on a plastic car bumper (to mount front plates) and the bumper was fine, however it was flatter than rocket tubing.

 

[MarsLander]

@JohnCoker and others use PEM nuts in aluminum and G10.
https://www.rocketryforum.com/threa...apped-in-fiberglass-sheet.182500/post-2495390

 

[David Schwantz]

I use rivnuts lots on the truck and trailer for sheet metal stuff. I don't think they would hold up on a rocket as the surface area after the crimp is not all that large. Also as they expand they would more than likely crush a cardboard tube and may even shatter glass. But I may be wrong, I was once long time ago.

 

[Dane Ronnow]

So you don't epoxy the well nut into the tube?

Nope. As the rail button screw is tightened, it's compressing the rubber well nut. Refer to photos #4 and #5 in my post. When the screw is fully tightened, the well nut can't be pulled out of the mounting hole. Like I said, it's rock solid.

 

[86mustang408w]

Myself I do the buttons during the build as I need to drill holes. If you use plywood or some other backing plate with the well nut, you can just unscrew them until your done painting. In that case I take a q-tip cut in half and stuffed down the airframe/well nut holes to stop the paint from going in or covering it up completely. You can stick a spare machine screw in the button hole and well nut too.

Or just use painters tape on the buttons around the outside before you primer/paint, that works too. You won't be able to properly do them without making a mess after its painted.

 

[SDramstad]

We had some students flying their level 1 certs a few months ago. One rocket hung up on the rail for a bit before it took off, hung for maybe half the motor burn time. The rocket didnt get very high and crashed before the ejection charge went off. When we recovered the rocket we noticed the lower rail button was gone. Checked the rail and sure enough we found it still in the rail. Pulled it off with some difficulty and saw it had a nice thick coat of paint on it. Rail itself was fairly clean and other rockets had used it before and after this flight with no problems. As far as we can tell the extra paint was what jammed it up. Never paint the buttons.

 

[SolarYellow]

Riv-Nuts are also available in aluminum, instead of steel. Should have a little less expansion force for application in composites.

When installing them, I like to use tooling that does not require the body of the nut to generate enough grip/friction on the sheet it's installed in to resist the installation torque. That can be a fancy tool or it can be just a bolt, nut, and some greased washers.

 

[Troy3003]

Riv-Nuts are also available in aluminum, instead of steel. Should have a little less expansion force for application in composites.

When installing them, I like to use tooling that does not require the body of the nut to generate enough grip/friction on the sheet it's installed in to resist the installation torque. That can be a fancy tool or it can be just a bolt, nut, and some greased washers.

Harbor Freight sells an inexpensive kit that includes the installation tool. Almost identical to pop rivet pliers. The kit I had came with aluminum riv nuts.

 

[jqavins]

I always have a threaded hole to attach rail buttons. This is typically threaded into CRs (if thick enough) or a hardwood block glued to the CRs.
This allows replacing buttons that got worn.

Then insert a screw, without the button and tape it off to do the painting.

I've only used buttons on one rocket, and that's what I did. I used wood screws into wood centering rings. Drilled pilot holes, put the screws in, painted, then took the screws out and installed the buttons. I think I cleaned the paint off of the screw heads. It worked out fine.

 

[SolarYellow]

McMaster has well nuts down to M3. Amazingly, I wasn't able to find those on AliEx.

From the McMaster link in the John Coker post above, you can also branch out and find carbon steel and metric PEM nuts. They are much less expensive on AliExpress.

Here's PEM's installation page:
https://www.trfastenings.com/knowledge-base/self-clinch-fasteners/nut-installation-and-tooling

 

[KenECoyote]

Is there any movement or flexing possible with well nuts given the construction includes rubber?

 

[waltr]

I've only used buttons on one rocket, and that's what I did. I used wood screws into wood centering rings. Drilled pilot holes, put the screws in, painted, then took the screws out and installed the buttons. I think I cleaned the paint off of the screw heads. It worked out fine.

I actually drill and Tap threads into the Wood Cr's. Most are 8-32NC.
After tapping the threads put a little Thin CA glue into the hole. It soaks into the wood and hardens the threads. Once fully set, run the tap into the threads to clean up.

 

[jqavins]

Why harden the wood with CA and then use a machine screw thread, when wood screws are made for exactly the this situation? No fussing with extra glue or running a tap in, and you even get a non-threaded shoulder for the button to spin on.

 

[Titan II]

Is there any movement or flexing possible with well nuts given the construction includes rubber?

No

 

[Dane Ronnow]

Is there any movement or flexing possible with well nuts given the construction includes rubber?

With the screw tightened, the fully compressed rubber is solid, with zero flex side to side or top to bottom. Grabbing the button with a pair of slip-joint pliers, you could probably get some lateral movement. But using the same amount of force, you'd get movement with a metal T-nut or rivet nut too.

The difference in those three methods is that the rubber well nut can be removed and replaced if necessary without tearing up the tube. Not so with the other two.

 

[Dane Ronnow]

Here's a video that shows how the well nut works:

 

[kramer714]

I use floating nutplates (day job stuff) MS21075-06 These are designed to be mounted to thin fiberglass and hold VERY well. You can install them without a jig, but the jig makes it easier. Two small countersunk rivets (nut plate rivets) hold the nut plate in place. We use THOUSANDS of these at the 'day job'.

The hole in the composite is 'just larger' than the major diameter of the thread, positions it but doesn't attach it. The 'float' make sure the 'nut' threads right, doesn't try and fight the threads.

top picture - nutplate used to mount a rail button
middle picture - a screwed in place coupler coupler with nutplates on top, bodytube screwed into a couple on the bottom
Bottom Picture - nutplates attached to the inside of the coupler.

Mike K