Riv Nuts in Fiberglass Top?

[fjefman]

I agree with the other comments. Rivnuts would be a bad idea, and VHB would be a good idea.

I have a 70 watt panel attached to the shell on the back of my Toyota Tundra with VHB. I basically attached small AL plates (with threaded holes in them) to the truck with VHB and then just screw the upright brackets for the solar panel into those pads. They have been up there since late 2014 without an issue. The truck has never spent a day inside in its life, and I live in GA so it has been exposed to all types of weather including snow.

If you PM your cell number I can text some pics.


Jeff

[gahamby]

Which VHB did you use? 3M has a lot of different tapes.

[fjefman]

Yeah, the joys of reading 3m Literature...almost as bad as reading charts from Loctite.

Here is the document I review (https://multimedia.3m.com/mws/media/...alty-tapes.pdf) and what I used was the 4949 series. It is made for high energy surfaces (aluminum to epoxy paint), has a medium thickness (.049") firm black foam, wide thermal characteristics and is available in a variety of different widths.

I actually find VHB tape to be very handy in the garage. I'm good at welding steel with my MIG, but don't have the ability (or skill) to weld thin aluminum. So, I just VHB it. Not for anything structural, but I have made some GPS mounts for my motorcycles and I've made a few hidden lockboxes for vehicles using AL plate, 90 degree angle aluminum for the "joints" attached to the plates by VHB. Sand the edges, paint and it looks and feel like it is one solid piece.

In the middle of making a lockbox now that will go under the Sienna seats of my van....again, plate, 90 strips and VHB...I use the 4929 series for these applications as the surfaces are perfectly flat and the tolerances are very tight.

Jeff

[Fool120]

I thought the less holes in the fiberglass the better, so I used VHB Tape with Marine Sealant around the tape, and at the edges. Later I added RV Tape to hold down cables. Looking back at my order: 3M RP45/GRY15 Scotch RP45 VHB Tape: 1" x 15 ft, Grey When warm the VHB tape grabs quick, and solid, so be ready for it, have the piece in the right spot.

Originally Posted by gahamby

Which VHB did you use? 3M has a lot of different tapes.

[BCam]

The VHB tape I have is 4950, which is similar to Jeff's 4929, except thicker. I'll admit that it seems risky to trust any kind of tape but everything I've read indicates that VHB is the real deal.

I haven't installed my solar yet since I'm dealing with some higher priority issues.

[fjefman]

As to thick vs thin, here are my thoughts. Where I know both pieces are absolutely flat (like AL plate to AL plate then I'll use the thin 4929. If the surfaces are just slightly off (like my solar mounts, flat AL plate to a slightly curved top) then I'll use the 4949.

The 4950 mentioned by BCam is exactly the same thickness and adhesive as the 4949 I used except it is a white tape, instead of black.

If there was even more irregularity I'd go with a thicker version like the 4955 or maybe even the 4959. Both of these are only available in white.



Jeff

[TomsBeast]

Don't use Riv-Nuts on fiberglass. They work by expanding themselves, and lock in by expanding the surrounding hole they are expanding into. That surrounding hole needs to be a malleable material like steel, aluminum, so the riv-nut 'locks in'.


Having said that, I've seen them used in fiberglass, which fractures while you expand the riv-nut, but they often spin, as was mentioned. No bueno.

I've used a boat builder's technique to make blind 'plugs' when I couldn't get to the backside, you drill a hole from the outside, and used a disposable plastic syringe to inject micro-balloon thickened epoxy paste, the consistancey of peanut butter. I've also bonded 1/4" thick aluminum plate, cut into 3" x 3" squares (w/table saw) with thickened epoxy, when I could get to the backside, then drilled an tapped into the aluminum, for a super strong fastening point, that spreads the load over 9 square inches. Bolts and fender washer backers are a pain in the neck for things like this, but an option. Again, IF you ca get to the backside.

The Gougeon Brother's published a booklet on boat construction that you can find on-line. It shows detailed information of several time-tested fastening methods for metal hardware to fiberglass. They give the information away because they sell and entire line of epoxy and consumables that I've used several times.


I hope that helps

[BCam]

On my previous travel trailers I had no problem with relatively small #8 stainless steel screws into the roof which was essentially 3/8" or 1/2" plywood covered by a rubber membrane. I'm sure their holding power was much less than VHB. According to this link, properly applied, 4950 VHB has a 140 lb/sq in. tensile strength:

https://www.interplas.com/3m-vhb-tape

A 2" x 4" solar mount at 8 square inches is going to be pretty hard to pull off.

[JWA]

Just a few tidbits from my experience with 3M VHB and riv-nuts........

Rivet nuts of the "split body" or "pre-flanged" types were actually made for thinner plastics as they work by widening the bearing surfaces they attach too. Spreading the load over a wider area in a clamping action makes them highly usable in fiberglass panels and so on. I use them in single or double thickness vehicle sheet metal as they're so far superior to sheet metal screws its amazing.

These go by many names "plus nuts" being one most here have discussed in the past I believe. They are well worth looking into for uses as discussed in their thread, I have a few photos of them before and after being "made up" to simulate their installation in blind applications.

As mentioned 3M's list of ALL VHB's is so extensive its difficult to navigate but info provided here relating to actual part numbers is excellent. I will echo the vast of available formulas is staggering. Add to that there is an "industrial" range AND an "automotive" range---if you're seeking 3M's amazing customer service you'll need to contact the specific division of the product you want to use.

Most of the formulas discussed here seem to fall into the "industrial" range---keep that in mind.

ALSO VHB is in use bonding metal panels to trailer skeletons during new construction replacing multiple rivets. While that isn't a structural application it should demonstrate its capability to bond and not fail when exposed to various environmental conditions.

Anyway---hope this helps a bit.

[Oly11]

Originally Posted by gahamby

Are they rated for exterior highway automotive use?

They advised not to use just these as a mounting source for a rack. I will have metal supports as well as a couple of these. I would think they could work for a solar panel but one should consult the manufacturer