Acrylic Thickness
- Thread starter 350xfire
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DIWdiver
Contributor
350xfire -
From Marks' Standard Handbook for Mechanical Engineers, the max stress in a disk supported uniformly around its edge is S=1.24wr^2/t^2, where:
S = max stress, PSI
w = pressure against the plate, PSI
r = diameter
t = thickness
Thus the max stress in your case is about 1.24(150)(6.5^2)/(1^2) = 7860 psi.
I got the flexural strength of acrylic as 100 MPa from:
Flexural Strength Testing of Plastics
From Pocket Ref, 1 Pa = 0.0209 lbs/ft^2
10^8 * 0.0209 / 144 = 14,500 psi
Confirmed here: Conversion Calculators - Convert MPa to psi- Chapel Steel
This gives you a safety factor of pretty close to 2. That's not bad if you're filling it with water and the consequences of failure are minimal. If you were to fill it with air, I wouldn't go anywhere near it.
Since acrylic is so sensitive to stress points, you should be very careful in your design and construction to minimize them. Annealing after machining may help too.
Interestingly, polycarbonate is listed as 90 MPa. However, PC is much less sensitive to stress points than acrylic. It's really a better material for this, but it's way more expensive at 1" thickness.
As a point of reference, the old Darryl Allen lights use a 3/8 or 1/2" flat plate on what must be about a 3.5" diameter o-ring. It might be interesting to know the exact measurements and depth rating and material of that light.
I just got done constructing my pot using an old alu 80, and I opted to use the bottom of the tank instead of a clear plastic. I was planning to pressure test it this weekend, but life got in the way. I'll be testing to 300 psi soon (rating of hoses and fittings I bought). It might accidentally go to 500 psi (;-).
What are you planning to use for a pressure source? I built a small screw-driven cylinder that I have tested to 600 psi. It seems much simpler and safer than the HP air my buddies have been suggesting.
D
From Marks' Standard Handbook for Mechanical Engineers, the max stress in a disk supported uniformly around its edge is S=1.24wr^2/t^2, where:
S = max stress, PSI
w = pressure against the plate, PSI
r = diameter
t = thickness
Thus the max stress in your case is about 1.24(150)(6.5^2)/(1^2) = 7860 psi.
I got the flexural strength of acrylic as 100 MPa from:
Flexural Strength Testing of Plastics
From Pocket Ref, 1 Pa = 0.0209 lbs/ft^2
10^8 * 0.0209 / 144 = 14,500 psi
Confirmed here: Conversion Calculators - Convert MPa to psi- Chapel Steel
This gives you a safety factor of pretty close to 2. That's not bad if you're filling it with water and the consequences of failure are minimal. If you were to fill it with air, I wouldn't go anywhere near it.
Since acrylic is so sensitive to stress points, you should be very careful in your design and construction to minimize them. Annealing after machining may help too.
Interestingly, polycarbonate is listed as 90 MPa. However, PC is much less sensitive to stress points than acrylic. It's really a better material for this, but it's way more expensive at 1" thickness.
As a point of reference, the old Darryl Allen lights use a 3/8 or 1/2" flat plate on what must be about a 3.5" diameter o-ring. It might be interesting to know the exact measurements and depth rating and material of that light.
I just got done constructing my pot using an old alu 80, and I opted to use the bottom of the tank instead of a clear plastic. I was planning to pressure test it this weekend, but life got in the way. I'll be testing to 300 psi soon (rating of hoses and fittings I bought). It might accidentally go to 500 psi (;-).
What are you planning to use for a pressure source? I built a small screw-driven cylinder that I have tested to 600 psi. It seems much simpler and safer than the HP air my buddies have been suggesting.
D
DIWdiver
Contributor
Here's another point of comparison -
Paul's Homemade Underwater Video Housing
All the way at the bottom shows a broken plate, and how it happened. I'm pretty sure that 'perspex' is acrylic.
D
Paul's Homemade Underwater Video Housing
All the way at the bottom shows a broken plate, and how it happened. I'm pretty sure that 'perspex' is acrylic.
D
mts0628
Contributor
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Hey 350xfire:
I think the 1 inch will be fine. Actually, that's what I'm gonna use as I have a 12 inch by 12 inch piece from McMaster Carr. In the Divelight Companion, Steve used a 1/2 inch piece, rounded it, drilled the holes, and put in in place. When he used it a couple of hundred times it finally failed. Not bad though. He has a neat picture of his setup on page 96- 3003 edition, and says that he is gonna switch to a 1 inch piece of acrylic.
The only thing I would add and I will, is a metal ring on top of the acrylic. This will better hold the acrylic and disperse any pressure from the securing bolts and lessen the chance that one bolt will cause failure (as seen in DIWdiver's link). Also in the link, the acrylic cover or front is stepped, which might account for the cracks (on the weak area of the acrylic). So, a maybe 3/16 inch mild steel ring fabricated from a sheet would do the trick for dispersion and strength of the pressure vessel top. And like DIWdiver suggested, fill it with water, it's safer and the things you're going to be tested need to be tested under "water pressure" anyways.
The other thing in the Divelight Companion is Steve put the gauge on the centre of the acrylic. I would rather plumb the gauge and fill port on the wall of the vessel which might reduce more of the weakening of the acrylic.
Good luck!
With kind regards,
Thomas
I think the 1 inch will be fine. Actually, that's what I'm gonna use as I have a 12 inch by 12 inch piece from McMaster Carr. In the Divelight Companion, Steve used a 1/2 inch piece, rounded it, drilled the holes, and put in in place. When he used it a couple of hundred times it finally failed. Not bad though. He has a neat picture of his setup on page 96- 3003 edition, and says that he is gonna switch to a 1 inch piece of acrylic.
The only thing I would add and I will, is a metal ring on top of the acrylic. This will better hold the acrylic and disperse any pressure from the securing bolts and lessen the chance that one bolt will cause failure (as seen in DIWdiver's link). Also in the link, the acrylic cover or front is stepped, which might account for the cracks (on the weak area of the acrylic). So, a maybe 3/16 inch mild steel ring fabricated from a sheet would do the trick for dispersion and strength of the pressure vessel top. And like DIWdiver suggested, fill it with water, it's safer and the things you're going to be tested need to be tested under "water pressure" anyways.
The other thing in the Divelight Companion is Steve put the gauge on the centre of the acrylic. I would rather plumb the gauge and fill port on the wall of the vessel which might reduce more of the weakening of the acrylic.
Good luck!
With kind regards,
Thomas
DIWdiver
Contributor
While driving home tonight I realized an error in my calculation. I entered the diameter of the disk where it calls for the radius. Thus the actual stress is 1965 psi, and your safety factor is 7.4.
This explains why Steve's survived so long (the 1/2" disk actually has the SF of 2).
I'm glad you asked this. I now have a lot more comfort about building the front lens for my light!
D
This explains why Steve's survived so long (the 1/2" disk actually has the SF of 2).
I'm glad you asked this. I now have a lot more comfort about building the front lens for my light!
D
DIWdiver, I'll just be using a plain air compressor. Probably puts out about 120psi actually. Yes, I will fill with water prior to pressurizing. Nice calculations there. You an engineer?
Thanks for the info guys.
Thanks again Thomas... BTW I'm still anxiously waiting on the lathe. May 25th can't come soon enough. Got most of the tooling already.
Thanks for the info guys.
Thanks again Thomas... BTW I'm still anxiously waiting on the lathe. May 25th can't come soon enough. Got most of the tooling already.
DIWdiver
Contributor
350xfire -
Actually, I am an electrical engineer, but I like to think of myself as a jackass of all trades.
I never realized how simple those calculations were until you asked. I've been wondering how thick my lens has to be to survive the 500 fsw test I plan to do on it (mostly for bragging rights; I haven't been below 130 ft in years).
Do you (or will you) have a steady rest or a follower rest? Without one, you may find that cutting 7" diameter 20" from the chuck is pretty challenging, and the consequences of the slightest failure are a little exciting
o). Or are you planning on a shorter pot?
They say the two happiest days of a boat owner's life are the day (s)he buys the boat and the day (s)he sells it. I don't think the same is true of lathe owners. I love every minute I spend on mine. I hope yours arrives as planned, and I wish you many happy hours on it.
D
Actually, I am an electrical engineer, but I like to think of myself as a jackass of all trades.
I never realized how simple those calculations were until you asked. I've been wondering how thick my lens has to be to survive the 500 fsw test I plan to do on it (mostly for bragging rights; I haven't been below 130 ft in years).
Do you (or will you) have a steady rest or a follower rest? Without one, you may find that cutting 7" diameter 20" from the chuck is pretty challenging, and the consequences of the slightest failure are a little exciting
o). Or are you planning on a shorter pot?They say the two happiest days of a boat owner's life are the day (s)he buys the boat and the day (s)he sells it. I don't think the same is true of lathe owners. I love every minute I spend on mine. I hope yours arrives as planned, and I wish you many happy hours on it.
D
350xfire -
Actually, I am an electrical engineer, but I like to think of myself as a jackass of all trades.
I never realized how simple those calculations were until you asked. I've been wondering how thick my lens has to be to survive the 500 fsw test I plan to do on it (mostly for bragging rights; I haven't been below 130 ft in years).
Do you (or will you) have a steady rest or a follower rest? Without one, you may find that cutting 7" diameter 20" from the chuck is pretty challenging, and the consequences of the slightest failure are a little exciting
They say the two happiest days of a boat owner's life are the day (s)he buys the boat and the day (s)he sells it. I don't think the same is true of lathe owners. I love every minute I spend on mine. I hope yours arrives as planned, and I wish you many happy hours on it.
D
SInce I don't yet know what I'm doing on the lathe, I was planning on running it to the local machine shop since they'll have a bandsaw big enough to cut it and a lathe with all the tooling to cut an o-ring groove etc.
What does your light look like?
And yeah, I hope that doesn't apply to a lathe. I owned a boat once!!! lol
Thanks again...
DIWdiver
Contributor
Yeah, there's a lot to learn. But it's more fun than school!
Right now my light looks like two circuit boards screwed to a large aluminum plate while I finish the firmware for it. Ultimately it is a handheld, all aluminum body, 1.5" dia x 7", with the front 1.5" (the head) widened to 2.1" diameter. Six Luxeon Rebels give it about 1100 lumens at 800 mA, powered by 8xAA NiMH. Optics are Polymer Optics 6 degree lenses.
D
Interesting. Let us see some pics when you're done...
Oh, and BTW, anyone looking for plastics needs to try out Allied Plastics. I found the 1" acrylic cut to a 7.25" circle for $22, they also have Delrin for $13 2.25", $17 2.5" and $28 for 3".
Oh, and BTW, anyone looking for plastics needs to try out Allied Plastics. I found the 1" acrylic cut to a 7.25" circle for $22, they also have Delrin for $13 2.25", $17 2.5" and $28 for 3".
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