# vacuum base for bell jar

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OK, I got a vacuum pump and I found a 1/2 gallon bell jaw at Fisher for \$42. I didn't see a base so I am wondering about making one.

I figure an aluminum plate would work fine. How thick of a plate do I need for a 1/2 gallon bell jar? Is 3/8 enough or do I need to go to 1/2 ?

thanks chuck

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If the 3/8 plate sucks up into the bell jar.....go to 1/2"

I should think the .375 would do the job.

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"Chuck Sherwood" wrote: (clip) How thick of a plate do I need for a 1/2 gallon bell jar? (clip) ^^^^^^^^^^^^^ Depends. If the bell jar is an inch in diameter and several feet tall, the aluminum plate would have to support only 11.5 lb, so it could be pretty thin .

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Thus, the birth of vacuum forming!

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But if it were a foot in diameter, then the force at the center of the plate would be about 1600 pounds or so. The height of the jar does not matter.

FWIW a commercial system might have a one foot dia bell jar and a half inch thick stainless baseplate. The baseplate has to be rigid enough to not oilcan excessively, because the innards of the thing being pumped down are bolted to it.

Jim

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"jim rozen" wrote: But if it were a foot in diameter, then the force at the center of the plate would be about 1600 pounds or so. The height of the jar does not matter. (clip) ^^^^^^^^^^^^ We're just kidding around, aren't we? So. I will point out that the OP specified that the bell jar is about 1/2 gallon. So, if it is a foot in diameter, it will be about 2" tall. Your force calculation gets the correct number, but the force will not be at the *center* of the base--it will be uniformly distributed over the entire area.

The only point I was trying to make in my facetious posting was that the required thickness of the base cannot be specified unless the diameter is known.

As a practical matter, a 1/2 gallon bell jar is probably about 6" in diameter, so it will have a force on it of about 400 lb. I would make the base out of whatever is available in the range of 1/4" to 3/4", just based on two things: 1.) You don't want it to "oilcan". 2.) You don't want it to be too heavy to handle easily.

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Be *sure* to put that bell jar in a steel wire cage to catch the particles if the base deforms and induces failure in the bell jar.

IIRC, the vacuum system that I used at work some years ago had a base of about 1" thick steel. The bell jar was about a 12" ID, IIRC. Yours sounds significantly smaller, but I think that I would avoid aluminum, and go for steel. And in *any* case, I would require the steel wire cage around the bell jar for safety.

Good Luck, DoN.

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I'd go with steel of 1/2" or thicker simply because I've never seen the base of a bell jar made out of anythign that wasn't thick steel. Al might flex and break your seal or chip the bell jar.

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Quite right, of course. I quite the entire force number often because it impresses folks, and makes them respect bell jars a bit more. The idea that it's a glass jar with two tons of force involved somewhere in there makes it easier to convince them that it has to be covered in wire mesh.

Ah, got it!

Sounds good to me.

Jim

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FYI, the bell jar is 6-7 inchs in diameter. thanks for the input. I will find some think plate.

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A 6" diameter disc of aluminum .25" thick, simply supported around the edge, with 415 lbs uniformly distributed over the disc, will see a bending stress of 2620 psi and will deflect 0.005" in the center. (Engineering Power Tools to the rescue :-).) 0.25" steel would deflect

0.0018", and 0.5" thick aluminum 0.0007".

-- Regards, Carl Ijames carl.ijames at verizon.net

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Why not just go with a dessicator bowl instead? That way you get the bowl, the lid, the method of sealing them together and the method for connecting it to your vaccuum pump. A half gallon size is pretty small. If you can get bigger, do it.

Pete Stanaitis

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Wow, Look like 1/2 inch aluminum (or steel) is the right choice.

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Didn't know it was an option!

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Sorry, my spelling ability failed me. It's desiccator. Search for "desiccator bowl" to get started. I found 4 or 5 hits.

Takes me back to the 60s thru the 80s when I worked in High vacuum technology.

Pete Stanaitis

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I have been following ebay sales of vacuum pumps. I noticed this bidder "mmetropolis" buys lots of vacuum pumps and some pretty big ticket items too. Makes me wonder what he is doing with them?

chuck

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