Conflat Ceramic Breaks Interest

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Liam David
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Conflat Ceramic Breaks Interest

Post by Liam David »

Sometime in the next month or so I'm going to place an order for some custom "zero length" 2.75" conflat ceramic breaks so that I can properly isolate my chamber. I'm curious if anyone here is interested in one. It's designed to seal against the sloped side of the knife edges with 131 size o-rings. Proper function as a break requires plastic bolts or some tricks with isolating metal bolts. Thickness is 1/4" but could be altered. Price would be something like $100-150 a unit.

ceramic break.png
RobertMendelsohn
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Re: Conflat Ceramic Breaks Interest

Post by RobertMendelsohn »

What an innovative idea! How are these made? Could plastic bolts really provide enough force to compress a gasket?

-Robert
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Rich Feldman
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Re: Conflat Ceramic Breaks Interest

Post by Rich Feldman »

>>What an innovative idea! How are these made? Could plastic bolts really provide enough force to compress a gasket?

Same occurred to me.
Molded nylon bolts have been a thing for more than 50 years, with well documented strength and temperature range.
Maybe they have a generous margin of strength for squeezing the O-rings.

Could we get a bit more strength-at-temperature, with bolts machined from fiberglass rod? Don't know about strength of cut threads in fiber reinforced composite materials. Might use fiberglass rods as big as will fit, and cut grooves for metal C-rings on each side of the CF flanges.

How about replacing the bolts with short, insulating alignment dowels? Then provide compression force with external C-clamps, or grooved half-rings like KF flange clamps, made from insulating material. Material stress could be much less than what's required in 6 bolts.

Sorry. Liam is probably way ahead of us on this!
All models are wrong; some models are useful. -- George Box
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Liam David
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Re: Conflat Ceramic Breaks Interest

Post by Liam David »

These are 99.8% alumina made by firing a blank and then machining to final dimension. I use the same manufacturer/material for my HV feedthrough which holds 100kV with no issue. I plan to use PEEK bolts which are stronger than nylon and most importantly have a much higher maximum temperature (260 C vs 60 C). I already use PEEK bolts for my feedthrough to great effect. Alternatively, I have also successfully used kapton tape around the threads of metal bolts and PEEK washers to isolate flanges.

Let's do some quick math (forgive the imperial units):

A 1/4"x28 PEEK bolt has a tensile strength of ~815 lbs (25 kpsi). This ceramic break is designed for 131 o-rings which have a cross-sectional width of 0.103" and circumference of ~5.9". Viton o-rings typically have a hardness of Durometer 75A. Looking at page 2-11 of the Parker O-Ring Handbook, 40% compression on a 0.103", 75A o-ring takes about 80 lbs per linear inch. Thus, our o-ring takes 472 lbs to compress, well within the capabilities of 6 PEEK bolts. On some flanges, I will have to make do with 3 bolts. As to why I chose 40% compression, consider Figure 3-8. Hint: I hate vacuum grease.
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Rich Feldman
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Re: Conflat Ceramic Breaks Interest

Post by Rich Feldman »

Thanks for tip about PEEK bolts, of which I was not familiar.
And here is a +1 for your reference to Parker O-Ring Handbook! An engineering bookshelf staple for decades. No need to guess when designing O-ring grooves for any geometric configuration and pressure.
All models are wrong; some models are useful. -- George Box
Matt_Gibson
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Re: Conflat Ceramic Breaks Interest

Post by Matt_Gibson »

I’d be interested in one of these.

-Matt
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Liam David
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Re: Conflat Ceramic Breaks Interest

Post by Liam David »

I should add that after putting in the order, the delivery time will be about 3 months.
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Liam David
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Re: Conflat Ceramic Breaks Interest

Post by Liam David »

So far I have 1 for Matt. I'm setting the deadline for additional requests to Sunday, March 10.
derekm
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Re: Conflat Ceramic Breaks Interest

Post by derekm »

use UHMWPE rope (dyneema) to compress the plates 6mm dia = 3000kg breaking strain. PTFE rod to locate. Its easy to splice.
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