Helicoil into Quartz?

[vc66]

Hi All-

I have a higher temperature (200-300 °C) application where I need to use a helicoil.

I've never tried to use a helicoil in quartz before, and I'm wondering if anyone has experience with it (holding power, strength, etc.). It's not holding a great deal of weight, but I'm getting some push back about using it. I've looked online, and in these fora, but haven't found any info.

Most likely I would be using an Nitronic Helicoil to prevent galling.

Any experiences or advice is welcome.

V

 

[zekeman]

Why don't you go to the mfr of Heli-Coil. They have a wealth of applications data.

 

[btrueblood]

I suppose it could be done...

Caveats which you probably well know - brittle materials are tough to thread, and don't hold threads well, the usual failure mode is for the first thread to fail in shear, and then the next thread, and the next, unzipping down the line. Using a helicoil may help to spread the load out, if for no other reason than the thread is size or so larger where the coil engages the quartz. Differential expansion will be an issue, but again adding a helicoil will help by adding some tolerance "slop".

No way you can make it a through bolt/fastener?

 

[IRstuff]

Can't imagine that there would be anything for the helicoil to engage against. Wouldn't it be sitting in a smooth-walled hole, as opposed to a threaded hole in metal?

Is it molded in place? The thermal expansion of steel is something like 30 times that of quartz. That means that whatever you've threaded into the helicoil would be under huge compression at operating temperature, possibly enough to cause some cracking?

TTFN

FAQ731-376

 

[GregLocock]

I think you'd have more luck cementing a threaded insert in place, using a nut and a bolt (preferred) or filling the hole with something solid that will take a thread and then drilling and tapping that, or using an expanding nut.

I like IRStuff's point about thermal expansion.


Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[amorrison]

Maybe you should consider using a "Hilti" anchoring system.
With an adhesive there would be no "abuse" of the quartz.

 

[KENAT]

Not quite the same but we use a lot of Granite here and our threaded inserts are secured with adhesive. However, we don't have the temp issue.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[ornerynorsk]

Potting with epoxy or other adhesive is the normal way to put in caulking anchors or thread inserts in stone products, as several others have mentioned in this thread.

 

[vc66]

Hmmm. All good suggestions.

Use of a nut is not possible, as we're trying to assemble/disassemble from one particular side.

I would be tapping into the quartz for the helicoil. So the hole would not be smooth.

My main perceived problem, as was mentioned, is thermal expansion.

Anything that I try to press, epoxy, thread, etc. into the quartz is most likely going to expand a lot more than the quartz.

The situation is that I have two pieces of quartz, surface to surface that I need to hold together. Attached is a cross-section of what I mean. Also, this will be in vacuum.

V

 

[KENAT]

Presumably you can't just put a thru hole in both quartz pieces and have a threaded hole/captive nut or the like in the material behind the quartz?

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[IRstuff]

Can you get screws made from Invar or something similar?

TTFN

FAQ731-376

 

[vc66]

KENAT-

The material behind the quartz is aluminum (no good for electrical isolation POV). Sorry should've mentioned that before.

IRstuff-

I suppose if they make 'em, we can get 'em. Does Invar have a low CTE?

V

 

[amorrison]

Wiki "invar" - good stuff

 

[IRstuff]

Invar was designed specifically to achieve a low CTE, so it should be perfect for this application, assuming Invar screws are generally available. I see that they are available for specific applications:

http://www.meteng.com/ceramnt.html

http://testing-services.globalspec.com/Industrial-Directory/invar

TTFN

FAQ731-376

 

[vc66]

Another perceived problem (news to me), is that the coefficient of friction between the helicoil and the quartz will be larger than that of the helicoil and the screw, so even if slight galling occurs, the helicoil will be screwed out of the hole.

Another reason why I was thinking of using either Inconel or Nitronic helicoils.

Any thoughts?

Thanks for all the help so far.

V

 

[amorrison]

Re: IRstuff

Your first recommended site meteng.com says the "strength" of invar is 30 million psi.

Does NASA know about this stuff ? (;<))

 

[IRstuff]

Actually, I believe that table is for the fused silica mount, and not the screws, per se. Not that the number is right, it's just not about the Invar screws.

TTFN

FAQ731-376

 

[IRstuff]

Oh, at the temperatures you've cited, Invar's CTS is about 1/3 that of Inconel.

TTFN

FAQ731-376

 

[KENAT]

How about a custom metal insert, perhaps invar as mentioned, of a shape that's fairly easy to machine into the quartz but resists turn out so the adhesive is just stopping it from falling out?.

Also, am I being dumb or do you have your coefficients of friction back to front?

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[vc66]

You're right, KENAT. I had 'em mixed up.

That's what I was thinking, but I have a feeling that will be shot down, as I imagine Invar is fairly expensive.

V