Acrylic Cylinder Strength


#1

Has anyone done any work with the Acrylic cylinder by way of using a thicker/larger/stronger tube for really deep diving version? I now that I can get 6" x 0.25 tube but not sure how much pressure it will resist.


#2

Hi Ronald:

Here at OpenROV HQ we’ve done a fair amount of testing of various tubes, both in our pressure chamber at the lab (which can go down to 200m), and at Lake Tahoe.

The 3/16" tubes that ship with the kit will consistently survive to 180-200m in our pressure chamber, so there is a decent amount of margin there in the rated 100m depth for the 2.8 ROV.

We’ve built a number of test ROVs using 1/4" thick tubes and thicker endcaps, and have dove them in Lake Tahoe down to 280m. The limiting factor seems to be acrylic endcaps- it is just very hard to make a flawless acrylic endcap that will not fail at those kind of pressures.

This spring we’ll be doing another round of deep diving testing at Tahoe, using endcaps turned from PVC, and some thicker tubing.

For you question specifically, I think 1/4" tubing will be good down to 300-350m. Deeper than that, I’d go with a 3/8" tube.

-W


#3

Just to add a few notes onto what @Walt_Holm said.

We did various testing of endcaps back with the 2.7 with different variables (not enough acrylic cement bonding layers, dirty pieces of acrylic, etc.) These survived down to the 130psi- 170psi range. What we consider “user quality good endcaps” survived down to the 200psi (~125m) range. In order to survive past this depth you have to have to have pretty flawless endcaps which is difficult to achieve. There is ultimately a limiting factor with the 6mm sheet of acrylic and some of the testing that Walt mentioned was with 9mm acrylic or other materials.

The yield strength of acrylic also varies widely and some of this is manufacturer dependent. These can be anywhere between about 5ksi and 10ksi. At a certain point the only way to get good data is to do actual testing.

There is a free piece of software called [Under Pressure] (http://www.deepsea.com/knowledgebase/design-tools/under-pressure-design-software/) that lets you easily do some of these initial design calculations.


#4

By the responses so far I’m hearing that it is the end caps that are the weakest point. Has anyone tried milled aluminum for end caps? I’m trying to build a pressure vessel for a 360 deg. drop cam system.


#5

Blue Robotics do aluminum endcaps for their 4" tubes (https://www.bluerobotics.com/store/watertight-enclosures/wte4-m-end-cap-r1/). Not sure what depth they have them rated at.


#6

Hi Ronald:

If you’re building a drop camera system, where things are going to be negatively buoyant, then aluminum should work just fine. Make sure you post some photos of what you do here, as we’re all curious to see what you’re creating.

-W


#7

Here is some hard won trivia about making watertight endcaps. Endcaps should generally be made from plate and not from round stock. Large round stock of many materials, especially plastics and aluminum, tend to have porosity at the center. If you cut a disk off of a rod you may find the disk leaks slowly right through the apparently solid material at the center.

The cause is actually quite simple. As the round stock is produced it is quite hot and expanded. When it cools it cools from the outside. Thus the outside becomes cool and rigid while the inside is still hot, soft, and expanded. When the inside cools it wants to shrink but with the rigid exterior there is nowhere to shrink from. So you end up with microscopic tendrils of vacuum running generally along the center of the round stock. When you cut a disk from the rod the tendrils of vacuum become pipes through the thickness of the disk. This is at the center of cooling which is not always exactly at the physical center of the rod. This does not happen with plate stock as when center of the plate cools the whole plate just gets slightly thinner.


#8

Dear Sirs
I have a spec sheet on acrylic. My question is if I elevate temperature can I form a head out of a piece of plate slowly with hydraulic press.
Also I would like to know if anyone has used pressure with temp to fuse a head.?
Is it possible with stress relieving ,preheat and post heat to weld acrylic. Has it been tried?
Is it possible to flange acrylic?
I’m thinking if I can get proper thickness . It’s possible to machine a groove on vessel and head. Then use a victaulic coupling. My major concern is coefficient of expansion and contraction of disinflation materials. It is achieved on pressure piping at these pressures.also I will be pulling a vacuum on mine.Any input would be greatly appreciated.
I only have experience with ASME section 8.


#9

Reynolds polymer has some experience with larger tubes and thicknesses. They were willing to sell me 8.5 OD tube. However when I shared my intent the possibility of liability scared them away. I’ve since changed my design. They suggested I try England.
Hope this helps on thicknesses. Thank you very interesting what is happening with this forum.