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Old 06-30-2017, 03:53 AM   #11
woundedpig
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Default Re: Unexpected response from 3M about VHB 4950 tape for rigid solar panels

I still don't get the 4 sq. inches per pound or load spec. It seems incredibly conservative. When I first heard about VHB, I went online and especially looked at youtube videos of industrial applications and demonstrations of sheer strength etc. These demos used nothing approaching the above spec of 4 sq in/lb load. VHB tapes are used in aircraft flying at tremendous speeds experiencing huge forces and heavy trucks bouncing down the road. I just looked at the aircraft and automotive applications sections on the 3M site.
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Old 06-30-2017, 04:12 AM   #12
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Default Re: Unexpected response from 3M about VHB 4950 tape for rigid solar panels

It has to do with that slow creep problem for constant static loads. Especially in shear. Things like picture frames. It is still a problem in static tension as well. You could hang a big old light fixture over your dinning room table and find it crashed to the floor the next morning.

Dynamic tension loading is a different matter - so much stonger! This is the kind of loading you are seeing on the truck trailer sidewall panels - if it doesn't have to hold the panel's weight up - ie the bottom edge of the truck sidewall is resting on a frame. In this case you'll see VERY much lower reccomendations.

They can't expect the general public to always know the difference so they are quoting you the shear static load number. That would apply if you wanted to stick panels on the side of your RV (without anything else supporting their weight.) Having them sit flat on the roof supported from below they are mostly going to experience tension loading some of which will be dynamic - from vortexes and some may be constant from angle of attack uplift as Autostartex calculated.

Last edited by rollerbearing; 06-30-2017 at 01:48 PM.
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Old 06-30-2017, 04:56 AM   #13
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Default Re: Unexpected response from 3M about VHB 4950 tape for rigid solar panels

What about adding a strip of Eternabond over the mounting foot, with say 1.5" of overlap on all sides?

I don't see what #8 screws into a FRP skin panel is going to do for you. I assume the roof core has essentially zero resistance to pullout.
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Old 06-30-2017, 07:12 AM   #14
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Default Re: Unexpected response from 3M about VHB 4950 tape for rigid solar panels

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Originally Posted by ablock View Post
...

I don't see what #8 screws into a FRP skin panel is going to do for you. I assume the roof core has essentially zero resistance to pullout.
Some ss screws might do more than you might think.

Is the roof plastic or composite - fiberglass? If composite the fasteners should lock in quite well. If plastic, not quite as well.

My thought is that the fasteners will clamp the parts together and help prevent the VHB or sealant from losing the bond.

I'm no expert.

I do like the idea of the Eternabond tape for weather seal. I doubt it will add much bonding strength.

I've mentioned before that Plexus adhesive has been used to bond boat decks to the hulls for quite some now. It is very effective.
http://www.itwplexus.com/UserFiles/F...To_Bonding.pdf


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Old 06-30-2017, 01:32 PM   #15
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Default Re: Unexpected response from 3M about VHB 4950 tape for rigid solar panels

Quote:
Originally Posted by ablock View Post
What about adding a strip of Eternabond over the mounting foot, with say 1.5" of overlap on all sides?

I don't see what #8 screws into a FRP skin panel is going to do for you. I assume the roof core has essentially zero resistance to pullout.
A bit better than screws are through bolts backed with load spreading plates. My fasteners on the interior are hidden in cupboards and behind ceiling AC vents. They are covered on the exterior with self leveling sealant. Agree with Vic about marine adhesives. Look up tensile strength of 3M 5200 and compare to VHB. Look up tensile strength of hardware store Permatex Silicone and compare to VHB.

Nonetheless, MANY people have used just VHB. If you go that route - really understand what you are doing - have plenty of safety margin (like 5X) and consider backing up with mechanical fasteners. The closest engineering data you will find is from the building trade - look up roof decking screw fastening schedules and wind uplift values. VHB is cheap and this is a case of a little seems to work and more is probably better (and sure wouldn't hurt anything or anybody.)

Last edited by rollerbearing; 06-30-2017 at 02:05 PM.
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Old 06-30-2017, 03:08 PM   #16
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Default Re: Unexpected response from 3M about VHB 4950 tape for rigid solar panels

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A bit better than screws are through bolts backed with load spreading plates.
I would say that would be much better, as in, that would be the marine industry standard. But also impossible in the Unity as the interior roof panels cannot be removed AFAIK.

5200 would be a good choic if you never, ever expect to need to remove the solar mounts.
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Old 06-30-2017, 03:28 PM   #17
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Default Re: Unexpected response from 3M about VHB 4950 tape for rigid solar panels

This is AMSolar's view: On fiberglass roofs they use VHB without screws with mounts that have just 2.5 sq inches of base area each. The Renogy mounts have twice that surface area each. 80 pounds of shear force per square inch of VHB = 800 pound of shear force using the AMSolar mounts, and twice that using the Renogy Z brackets. I tend to over analyze things and I'm in that phase right now.......... Still pondering. I don't know how thick the fiberglass shell is for the Leisure Travel Unity and don't know exactly what is underneath (4 inches thick) but wonder what the resistance to screw pullout really is, like Ablock.

I just heard from Garret, an engineer at AMS. He said that 3M's response looks like a "CYA" to him and again, no failures. He suggested adding an extra mounts when the panel is mounted near the leading edge of a vehicle where there might be laminar airflow and said that AMS tends to add extra L-Feet to increase the contact area in such a setting.
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Old 06-30-2017, 04:33 PM   #18
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Default Re: Unexpected response from 3M about VHB 4950 tape for rigid solar panels

There are so many issues at play here... if you simply #8 or #10 sheet-metal-screw it to the fiberglas, i'd really be worrying about pull-out (if the VHB failed/crept). But you'd certainly be helping in the straight-to-the-rear shear issues.

Next up would be to drill a bigger hole, and insert/install a Molly-like T- or expansion "nut" ... there's a WingIts brand for ADA handrails which uses two coupled broad-spreading plastic tripod back supports ... good enough to exceed 600 pound support in 5/8" wall board, so quite likely decent for fighting lift effects of a panel.

A problem with putting any hole in the fiberglas is the potential for fatigue/stress cracks spreading from it as the roof flexes up and down from the winds buffeting the panel. Bigger, smoother holes help minimize that.

Me? I'd accept the minor cosmetic damage of cutting through the ceiling inside the van to reach one or two of the forward feet to provide a load plate (my reason for saying "bolts and nuts" earlier... i'd certainly include load-spreading fender washers (at least)). You can always hide the ceiling hole with a new LED light fixture or the like.

--dick
p.s. when i installed my MaxxFan, i went the route of installing a wooden frame beneath my steel roof to receive its mounting screws, instead of just trusting the 12 or 16 screws' threads in the sheet metal alone. Belts and suspenders.
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Old 06-30-2017, 04:37 PM   #19
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Default Re: Unexpected response from 3M about VHB 4950 tape for rigid solar panels

Based on boat experience I would make wooden or aluminum mounting pads, glue them down with 3M 5200 polyurethane and screw or bolt the panels to the pads.

Using 5200 you can probably lift the van by the pads! That stuff sticks.

And if you ever have to remove them for some reason you can cut them off with a length of thin wire fishing leader
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Old 06-30-2017, 05:37 PM   #20
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Default Re: Unexpected response from 3M about VHB 4950 tape for rigid solar panels

Math vs. REAL WORLD:

My Renology solar panels are attached with VHB tape (don't know which number) for a couple of years now and have been from Wisconsin to Texas to California and Alaska without as much as a whimper. I do go up on the roof every once and a while to check, but for my money, no penetrations is a good thing.
I understand the math and the concern, but let's face it folks, the VHB tape just works.

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