Instructables

How much pressure can the average milk crate withstand?

Can the average milk crate (actual milk crates, not the cheapie wallmart 'back to school' crates) withstand at least 20PSI on all sides? I'm intending to use one as the support structure of a vacuum chamber.

kyuubiunl3 years ago
As a suggestion. Try a small cube made of your plexi first. Make sure it is a decently thick piece. Come to that, you should be able to find some engineering data about your specific material and its pressure properties. Take your cube, use a chemical bonder (chemical welding, think PVC glue) on every joint, and use screws to pull it together. Now the fun part. Tap and die two 1/4" holes into the cube. Buy a cheap vacuum gauge (1/4") and put it in the first 1/4" hole w/ teflon taped threads. The second hole needs a 1/4" M x M coupler. Teflon tape the cube side well and screw it in. Remember, tight enough is TIGHT ENOUGH. Don't over tighten and fracture your material. More torque, a better seal does not make. Buy a cheap 4 cfm small appliance vacuum pump (sub $100 / ebay) and a 1/4" refrigeration hose (800 PSI Burst, 400 PSI sustained rating should do it). Vaccuum! (*Note: The top of the box with connections can also be made to have a decent foam seal and bolts so that it can be removed)
kevinhannan3 years ago
I haven't read all the answers - sorry.

It also depends on whether your pressure is quick or slow.

Some plastics fracture under quick, intense pressure while a slow pressure would not break the crate.

Why not get a spare crate and test it for your conditions?
Well they're pretty tough, but not in all directions.  They're much weaker diagonally and usually have a weak point in the center of the bottom of the crate as well. 

I would think that the answer is no.  20psi generates a lot of force on that kind of surface area, and the milk crate was designed for a much different kind of workload.  Still, if it's all you have and you have a safe area and the means of testing it... by all means try it.

Give it a wide berth and situate it or yourself behind a sturdy concrete, brick or stone wall.  If you have many, test a bunch until breaking point.  Whatever level it fails at, divide that by 5 and call that your safe operating level.

Anyway, I'd think that a welded steel cage would be much stronger, or better yet an actual vessel designed for vacuum pressure.

Im sucking air out of it(it will become an airtight space) all I need it to do is support the weight of the atmosphere pressing on it.

Besides, I have a double layer denim apron, a little plexiglass shards wont be too dangerous.
I personally wouldn't trust an apron for this.  Ork's calculations do point out a serious problem, and that measurement is only for ONE face.  The math gets much more complicated when you add the other five faces and one happens to be of a different construction.

I've imploded a couple smaller plastic containers, including a ridged plastic water bottle.  That sent a piece of plastic straight through a 3/8" plywood wall.  That was done with a kludged-together pump, and nowhere near 20psi.  You're taking a much stronger container under much more stress, but expecting it to hold up to extreme abuse and NOT injure you?  Isn't that a lot to ask of the milkman?

Take a little time to think of a way to safely try it.  I encourage testing, but you really should put something between you and the
Isn't atmospheric pressure around 14psi?

that means if you create a perfect vacuum in the crate you still only have 14 atmospheric pounds inside.

100% agree with all other points.
Right you are. 14.696PSI.   Boy is my face red.  I was going along with the question without even thinking about that.  20PSI wouldn't be likely unless he's putting the vacuum chamber underwater. 

Still, a near-perfect vacuum would create more than two tonnes of force on each face.  He's unlikely to create something with such efficiency, so let's just say he makes it to a difference of 10psi.  That's obtainable and still creates 1960 pounds of force on each face of a 14"x14" cube.
I thougt it was very close to 15PSI, but I didn't know for sure. Add that to the fact that I'm accident prone, I added 5PSI onto that as a safety factor. If it'll withstand 20PSI, it'll surely withstand 15.
Safety factor...good thought.  Either way - a milk crate's not gonna make it.
The Ideanator (author)  frollard4 years ago
After I thought about the implications of a 100% vacuum(not possible for me, the strongest thing I have is a shopvac), I figured it wouldn't probably hold up. I'm thinking about saturating a cement tube in silicone calking. I was just wondering if anyone who trolls the questions section knew what kind abuse a milk crate could take(aside from its intended use).
orksecurity4 years ago
I'm betting the answer is "no". I realize that isn't the answer you want, but think it through:

Atmospheric pressure adds up. 20PSI on 144 square inches (assuming a 1 foot cubic milk crate) means almost a ton and a half on each face. (2880 pounds). I've broken the bottom of a milk crate by standing on it. Admittedly my weight was concentrated on just a few square inches, but ... well, you're talking about the weight of a car; even when spread out, that's nontrivial.

If the crate's face is larger, the force is too.

Feel free to try it, but I agree with Guardian Fox that sudden catastrophic failure is a plausible scenario and two layers of denim (a) may not be enough and (b) will do nothing to protect your face/head/neck in any case. You're dealing with an implosion risk here, which means the possibility of throwing shards at high speed if anything is sufficiently fragile to fracture rather than simply crumple, or if it simply recoils strongly when the vacuum breaks.

That's all assuming you can actually make a seal which will withstand a 20PSI difference. I'd expect your joints will shred themselves before the crate does --- but I wouldn't bet my safety on it either way. I second the suggestion to assume this is dangerous until proven otherwise, and to build in a serious safety buffer.

Sorry... but that's the best answer I can give you. I seriously doubt that there is such a thing as an "average milk crate", or that any of them have been tested for this usage, so you either need to find out a lot more about how yours is constructed and do some serious engineering calculations, or assume the experiment is dangerous until proven otherwise -- and even then, progressive failure remains a risk.


So... Doesn't the atmosphere naturally put about 14 psi on our bodies? How many cars is that (for a husky boy)?
Remember that this is about a pressure difference, you're pumped up to ~14 PSI on the inside.

L
I ALWAYS KNEW I WAS PUMPED UP!!!
Normally, that's balanced by internal air/hydraulic pressure. (What, you've never done the "watch air pressure crush a metal can" experiment?)

Valid point about 14 rather than 20. The rest of my comments, I think, remain valid, just scale appropriately. It's still over a ton on each side.
lemonie4 years ago
You'd be better with something round. But I'm struggling to think of an alternative that isn't a steel cylinder...

L