Crush depth of tubing


#1

Assuming that you actually want to carry out a test to destruction (we do) then the weakest part is obviously the sealed electronics tube (10mm) diameter. Using a sealed 100mm diameter pipe we intend to either lower the pipe into the Aegean sea to a depth where it physically implodes (or cracks), or to do a lab practical (we have a lab but no tools...)

At this stage we wouldn't care to comment on damage to mechanics at depth.

Anyone have any input?

Alex


#2

Just to clarify a bit, the sealed electronics tube is 100m of diameters, not 10mm. But yes, I think the tube is the thing that is most subject to cracking, expectantly if extruded and not cast.

Maybe a thicker wall will help, but, let us know of your findings.

Simone


#3

How about pre-pressurizing the tube? I guess that could be fairly easy with a scuba bottle/connector and a valve. I guess this would mean the electronics coming under pressure that they wouldn't have before.

Another idea may be to add a strengthening ring inside the tube, or, how about a slightly smaller tube inside the main one, with an area cut out for the camera to look through.

For the main ROV design, maybe you're doing this already, but it would seem like a good idea to permanently seal one end of the tube, leaving only one end to deal with the problems of o-rings/etc.

Another idea, stolen from the sperm whale, would be to fill the entire tube with molten wax, which would then harden, provide pressure and water resistance, and could even be removed by warming up the whole thing. I guess it depends on whether you need repeated access to the internals, and how your camera moves.

Great project!


#4

According to quick simulations the 3mm thick tube will only withstand about 50m depth, i am curios what kind of calculations the open ROV team has made to be able to claim the 100m working depth.


#5

No idea... it has the electronic chassis inside that acts also as reinforcements. Maybe that will help.


#6

Anymore info on these simulations please?

Mathematical formulae?

Computer program?


#7

I used the one linked to here on the forums: "Under pressure".

Sure thing it might not be 100% accurate, but should not differ 50m to 100m?

Just curious ; )


#8

Some links here for pressure calculations: http://openrov.com/forum/topics/pressure-calculations

If you do the empirical test, please take lots of photos! :)


#9

Any news on this? What calculations did you guys use when you promised 100m depth?


#10

#11

I think its harder to keep pressure inside a can than outside it. So I wouldn’t pre pressurized the can.
Easiest in my mind would be to use a thicker tube.
Though if you want real depths oil compensated is the way to go. But then you thransfere the pressure inside, so capasitors on the processor board and the camera is going to be a problem, they don’t like oil and pressure


#12

I saw a thread about a Mares dive computer that had a silicon covering over the circuit board, and that was about it.

How about covering the circuit board with a thick layer of silicon, and then filling the whole tube with wax? The silicon would keep the parts free of wax, and the wax would basically make the tube a solid tube of wax, which should cope with decent depths.

I guess a thicker tube would be easier, and anyway the space for the camera to look out would provide a weak point in the wax idea.

Actually there is no need to have the entire tube see-through - wouldn't it make more sense to have a metal tube (I expect that would be cheaper than a thick acrylic tube), with a thick dome-window on the end? Inside that could mount one of those cctv cameras with 360degree movement. I guess that would be for another project..!


#13

You could also upgrade to tempered glass.


#14

Ok random idea here, how about cannibalising a Pressure Cooker?! Remote Operated Cooker!


#15

To claim 100m of water depth we need to have minimum 6mm of thickness having ID of 95mm....I am working on it will post the calculations later...


#16

Did you calculate with a 200% safety margin then? Cause if you want to promise something you need at least a 200% margin considering no tubes are perfect.


#17

Factor of safety of 1.5. Means designed for a crushing depth of 150m. For pressure cases it is more than enough....


#18

Ok, just wanted to make sure : )


#19

The weakness in your design is the flat sides on your electronics tube.

Have you looked into vacuum forming two half spheres out of 10mm perspex?

if you leave a lip on both halves you can 3D print up a rubber C channel O ring to fit around the outer edge then use a ring type clamp to hold it all together.

As the pressure rises the rubber should be forced into the gap between the two halves witch should increase the seal.

Wiring that needs to be passed from the electronics bay to the outside is the real problem, no matter how it is done it will weaken the hull integrity. If all wires are packed into a threaded tube filled with silicone and reinforced with polymorph, Drill a hole through the hull fit a rubber grommet insert the tube then use more polymorph to make an inner and outer plate to hold O rings against the inside and outside of the hull, then hold it all in place with nuts


#20

We've added thickness to the lip of the end cap to increase its strength. The failure mode of the endcap lip breaking and the main tube buckling now seem to be at about the same pressure- something just past 1000kPa, which is the specified maximum depth of the ROV. To keep costs and building effort down, I've been trying to keep things as simple as possible, but it might be possible to extend the failure depth slightly (for both the end cap and buckling failure modes) by adding inner braces to the main tube which would keep the end caps from pushing inward and would also add rigidity/ reduce effective length to the main tube.

Thanks for the advice- and please keep in coming!

Eric