Thruster motor for underwater AUV/ROV

[Tinker12]

Just wanted to share my experiment on how cheaply I could make a nice underwater thruster motor for AUR/ROV project I'm thinking about.
The prop design is scaled of a stl file I found online to fit a film can size housing. The duct, prop adapter, and internal o-ring parts are my own design.
Kids around the country make a simpler version of this film can thruster for a marine engineering competion using a SeaPerch ROV. Just wanted to see if i could improve
on the design. see photos at https://www.dropbox.com/s/64yy001x4wqd6 ... 1.JPG?dl=0,
https://www.dropbox.com/s/pnsv8495musvx ... 2.JPG?dl=0
https://www.dropbox.com/s/q509fypss8jdg ... 3.JPG?dl=0. The motor is 2-20vdc and I plan to have
3 to 4 of them for the vehicle. I'm testing to see how well the o-ring motor shaft seal holds back the water, had a slight leak at first but seems ok now after
adding some toilet bowl wax after the o-ring. The cost is about $5 thanks to the wonder of Orion 3d printer.

[doctorgonzo]

That's pretty cool!

Have you considered printing in ABS and smoothing with acetone vapor?

I would tighten up the clearance between the impeller and shroud as much as possible.

[Tinker12]

I have never printed in ABS as my goto material is T-glase, I was concerned about ABS fumes since my printer not enclosed and in my living quarters. Smoothing out I something i might try, I remember seeing something online about a coating used on T-glase that makes it smooth and transparent.

[Captain Starfish]

Damn, that's cute!

Agreed on closing the gap between blade tips and shroud. The point of a shroud is to eliminate tip vortices and if they're not almost touching the resulting turbulence tends to make it worse than not having a shroud in the first place.

Increasing the resolution of the model on the circle of the shroud (or everything, really) combined with an ABS print and vapour bath afterwards (or the T-Glase treatment if you can find it again) would really help this.

Dive shops will sell little pots of silicone grease. Absolutely the go for marine application o-rings, used on camera controls, dive gear, just about anything and will work a lot better than your dunny seal.

What kind of depths are you going to run this to, do you think? Have you pressure tested it yet?

[Tinker12]

Well, as far as running depths go I was going to start with the local rec center pool 10-12 ft max. Have not really pressure tested it beyond my kitchen sink. Think I need some kind of pressure cooker modified for more testing.

[McSlappy]

Awesome print mate!

[Tinker12]

Thanks for the compliment!, Does anyone know what the best o-ring material is for this application, i.e. marine application o-ring material?

[Captain Starfish]

You have some choices. Simplest is an o-ring, I'd go for a bog standard 70 duro (this describes the hard/stiffness of the rubber) nitrile rubber o-ring. I would be inclined to try for two separate o-rings along the shaft, each in its own groove in the stuffing box.

More serious would be a proper stuffing box with a cylinder that has a hole down the guts for the prop seal and an end nut that bolts in and compresses the seal block against the walls of the stuffing box and the prop shaft so you can find that magic compromise between sealing efficiency and friction.

Even more serious would be to find your local industrial seal supplier and look at their collection of rotary seals, but I doubt you'll find anything small enough there.

My pressure testing rig consists of a home depot style 10l pressure pot for a spray gun hooked onto a compressor. Runs down to about 300 feet before the seal on the pot starts leaking. Pressurise it, let it soak for a couple of hours, then rapidly depressurise it and pop your test item into the water. If it's leaking you'll see the gas that got into it trying to get out again as bubbles - and this way you don't discover a leak by seeing your valuable electrics all flooded!

[jdurand]

A couple of ways we leak-tested in the MIL-SPEC business:

1) Place electronic part in a tank of hot FC-48 (high-temp fluorocarbon) and look for bubbles. Doesn't bother the electronics, actually cleans the board.

2) Place in chamber and pressurize with helium. Pause. Pump out helium, purge, and then test for helium leakage. VERY sensitive to the smallest leak.

[Captain Starfish]

Nice if you have access to the materials and gear!