Having trouble fastening my clear, acrylic bubble cover

[bitbrain]

Hi ROVers. I had a problem last summer with my ROV that prevented me from making my maiden voyage. I've been scratching my head ever since and trying to come up with a better strategy. It suddenly occurred to me to ask the community for opinions.

Note: attached are a few images.

My ROV hull is made from PVC plumbing parts. A large, 4" PVC tube for the main structure. I used a toilet flange on the end where the lens cap is to be fastened. I made a custom-cut rubber gasket that would go between the toilet flange and the acrylic lens cap. I drilled additional holes in the acrylic lens cap so I could have more places to tighten the lens cap with bolts and screws (to ensure leaks won't occur). I realize that I shouldn't over-tighten the lens cap, and I tried not to. But a couple of hairline cracks started forming on the acrylic lens on the newly drilled holes when I tried assembling (probably due to tension on the new holes from the bolts). I tried gluing the cracks with special acrylic glue (that "melts" the acrylic back together), but I have my doubts that will be good enough to water-proof my ROV down to ~70 foot depths (freshwater).

Additional note: The bolts that I used for fastening the lens cap to the toilet flange are not perfectly straight. A few of the bolts are at slight angles. This is worrisome, since this means the pressure of the bolt head will not be uniform around the acrylic holes. Furthermore, slight flexing of the acrylic dome base is noticed when I tighten the bolts a little. Either the acrylic lens cap is too thin to be used in this manner, or I am over-tightening the bolts. But I don't think I over-tightened.

Would really appreciate hearing the communities input. I'm open to other ideas, but I think these are my options:

1) Push ahead with this strategy. Either using the existing dome lens cap or maybe replace with a new one.

2) Ditch the acrylic lens cap, and opt instead for a thicker (between 1/8" to 1/4"), flat, piece of clear acrylic (for the lens cover). <-- This is my favorite option at the moment

3) Buy a really large metal or PVC washer to place around the lens cap. Drill holes in it for my bolts. This way, the head of the bolts won't create uneven pressure points on the acrylic lens. <-- This might be good to use with option 2 as well.

4) find some sort of C-shaped clamps than would hold the lens onto the toilet flange (pinching it closed, without using bolts). But I couldn't find anything suitable like this.

Thanks in advance for your thoughts.
Sincerely,
BB

[Bennachie]

Hi, your big metal washer idea is the first thing I'd suggest. That's how acrylic covers are attached on commercial ROVs.

Secondly, don't over tighten the bolts, you only have to tighten them just enough to create a seal with the gasket. Once you are over a few feet down, the water pressure holds the whole thing together.

[bitbrain]

Hi, your big metal washer idea is the first thing I'd suggest. That's how acrylic covers are attached on commercial ROVs.

Secondly, don't over tighten the bolts, you only have to tighten them just enough to create a seal with the gasket. Once you are over a few feet down, the water pressure holds the whole thing together.

Thanks for the helpful feedback.

[bitbrain]

Does anyone think I can improve the seal between surfaces by applying marine grease (or similar)? Or is it better to keep these surfaces clean and dry, and let the pressure-fit do it's job?

Surfaces that are coming together:
- Acrylic lens
- rubber gasket
- spray-painted PVC

[Bennachie]

Grease won't actually add much at all to the seal. Best to get a good fit using dry components for gasket seals. Grease is good for o-ring seals, and certain waterproof connectors.

What's important is the finish of the various surfaces and the hardness and thickness of the gasket material. Your PVC would be better un-painted but you are where you are. It might help to give the painted face a light sand with very fine sandpaper on a flat board.

More importantly, you might want to use a softer rubber for the gasket.

Most important of all, you want the acrylic not to deform and crack, so the big metal washer is a priority.

[bitbrain]

Grease won't actually add much at all to the seal. Best to get a good fit using dry components for gasket seals. Grease is good for o-ring seals, and certain waterproof connectors.

What's important is the finish of the various surfaces and the hardness and thickness of the gasket material. Your PVC would be better un-painted but you are where you are. It might help to give the painted face a light sand with very fine sandpaper on a flat board.

More importantly, you might want to use a softer rubber for the gasket.

Most important of all, you want the acrylic not to deform and crack, so the big metal washer is a priority.

Excellent! I appreciate your advice.

[Bennachie]

For gasket material, try fluorosilicone sheet. That's what we find most effective. Maybe the 3.2mm stuff. It's expensive but maybe worth a shot.

[Bennachie]

Also, I can't see what the sealing face of your dome looks like, but if it's anything like the opposite face I'd maybe suggest a light sanding with very fine sandpaper too.

If you do sand acrylic, you have to do it very slowly as it quickly heats up and begins to melt and clog the paper.

[bitbrain]

For gasket material, try fluorosilicone sheet. That's what we find most effective. Maybe the 3.2mm stuff. It's expensive but maybe worth a shot.

I did some research on your recommendation for fluorosilicone, it sounds like a really good idea.
If I got an O-Ring of that material, it would *just* fit between the screws and the opening, please refer to my attached picture. But I wonder if a round 1/4" gasket will be sufficient...a sheet of the material could make a better gasket, for sure.

Amazon: 346 Fluorosilicone O-Ring, 70A Durometer, Round, Blue, 4-1/8" ID, 4-1/2" OD, 3/16" Width
https://www.amazon.ca/346-Fluorosilicon ... ay&sr=8-24

[Bennachie]

You could... but as you plan to go to about 70 feet depth, the external pressure on the o-ring will be just over 2 atmospheres, or about 30psi above the that inside the hull. There's a good chance your o-ring would not withstand that and it would just pop inside the hull. You could add a lip just inside the hull to stop the o-ring slipping in, or even better, get a groove machined into the flange to take the o-ring.

If you don't machine a groove you also risk breaking the dome by over tightening the bolts. The best and probably easiest solution is to buy a small sheet of fluorosilicone and cut a gasket.

*Edit* I t looks like you don't have room to machine a groove so that leaves adding a lip or making a gasket.