non-permanent sealing of screws

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AllenWallace
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non-permanent sealing of screws

Post by AllenWallace »

I need to put several low voltage feed-throughs in my vacuum chamber, which is a modified bell jar design.
These capacitor-style feedthroughs use a 8-32 thread and I've drilled and tapped the chamber. I can epoxy these in, but I rather not in case I have to replace one.
Does anyone have any suggestions how how to seal this in a way so that I can remove the screw if needed?
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Richard Hull
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Re: non-permanent sealing of screws

Post by Richard Hull »

I assume you will only be feeding very high frequency signals >1mhz through these and not any DC voltage or frequencies under 5khz? Max voltage on one of these might be 1 kv RF. NO DC power though.

If you realize they are only for RF, then I would seal them using a simple vacuum grease. A flexible Epoxy and even RTV would also be good. The RTV and epoxy would have to be "cured out" well though as their volitiles can take a while to pump. RTV is bad for a pump due to the acetic acid content until cured or outgased.

Should you need to replace one, ( I assume it is toast), just take a torch and heat it to near red heat and the epoxy/rtv will release.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
AllenWallace
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Re: non-permanent sealing of screws

Post by AllenWallace »

This is a follow-up to sealing screws... Using just vacuum grease didn't work at all. The vacuum just ran through the threads. I could even feel the vacuum pull with my finger on the vacuum wall exterior.
I ended using Teflon tape with the 8-32 crews. The chamber was professional leaked detected and it passed with flying colors.

FYI, the Leak Detector company recommended using Teflon tape and plumbers Teflon sealant (goop) to seal screws.
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Rich Feldman
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Re: non-permanent sealing of screws

Post by Rich Feldman »

Richard, you might have missed or forgotten something.

In my limited experience, traditional "feedthrough capacitors" generally have a solid metal DC path from one end to the other.
There's also a controlled capacitance between the signal path and the surrounding sleeve.
Commonly, the bulkhead that's fed through is a metal RF enclosure.
The idea is to pass DC and low frequencies, without letting very high frequency stuff get through the wall.
Look at the insertion loss table in this listing: http://www.mouser.com/ds/2/394/SFBCC-26171.pdf

A more general term is "feedthrough filter", including ferrite beady feedthroughs etc.
There are a bewildering number of physical and electrical options in this catalog: http://www.tusonix.com/4000R7Sec1.pdf
from, you guessed it, Tucson, AZ.

I had no idea those things were so expensive to buy new!

Allen:
I'm surprised & pleased that the teflon tape worked with non-tapered threads. Do you think the seal comes from axial compression of the teflon against the bulkhead, on the side opposite the threaded stud and its nut?

As an alternative to sealing the threads with goop that hardens, I was about to suggest using solder-type feedthrough capacitors. They could be placed in holes in a sheet brass or tinplate diaphragm that's "permanently" epoxied over a large hole in your baseplate, or on the ends of short bits of brass modeler's tubing. Then replacing a feedthrough would be a quick and easy operation with soldering iron.
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John Futter
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Re: non-permanent sealing of screws

Post by John Futter »

Allen
Rich has it right on the DC path
I would use ordinary araldite (the 24 hour one) to seal them in

If you want to remove later just heat the whole assembly to 150-200 degrees celsius and the epoxy softens.

I would tap the holes to the same thread form to minimise the amount of expoxy and maximise the path length the leakage molecules have to go through
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Richard Hull
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Re: non-permanent sealing of screws

Post by Richard Hull »

I did, indeed, forget that the feed throughs were direct. The valued capacitance is just between chassis and the feed through wire. Sorry 'bout that and thanks for catching my faux pas.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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