Solderless Grid Design with Shaft Coupler

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Nathan Marshall
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Solderless Grid Design with Shaft Coupler

Post by Nathan Marshall » Fri Apr 17, 2020 1:17 am

I finished making a new grid today, and I thought I would share a few pictures of my design. This was fairly simple to construct. The most time was spent forming the tungsten wire into nice circular rings. I accomplished this by bending the wire around a 1" PVC pipe tightly. Once I let off the tension, the coil sprung back out a bit so the diameter was closer to 1.5". I then snipped the coil into rings with about a half inch overlap. I used this overlap section to bend out the little angled parts shown in the pictures below. I then spent some time cautiously bending the rings here and there to make them as flat as possible since they are helical when they are cut off the coil. The angled parts of the rings were then put into a small crimp sleeve made out of thin copper sheet. I slowly crimped the sleeve down, stopping to readjust the positioning of the rings as I made the crimp tighter and tighter. Once the rings were reasonably snug in the crimp sleeve, I placed the stem into a 1/8" set screw shaft coupler that I got from servocity.com. I tightened the set screw down to snug up the crimp even further. Now the other end of the shaft coupler can be directly attached to the feedthrough of my fusor. I also made a smaller 1" version by winding the tungsten around a 5/8" steel pipe. The smaller version is pictured to the left of the larger version in the shaft coupler.
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Rich Feldman
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Re: Solderless Grid Design with Shaft Coupler

Post by Rich Feldman » Fri Apr 17, 2020 1:33 am

Very nice. Is your wire pure W or some refractory alloy that's less brittle?

Let us know how it works out.

Has the forum seen reports of connections like yours becoming loose after many thermal cycles? It might help if the clamping had more elastic compliance than set screw in a stiff bushing. Springy parts must not yield or creep under stress at their operating temperature.
All models are wrong; some models are useful. -- George Box

Dan Knapp
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Re: Solderless Grid Design with Shaft Coupler

Post by Dan Knapp » Fri Apr 17, 2020 2:33 am

I have used a similar design with a set screw sleeve having six holes (parallel to the bore). Using tungsten wire loops, the originally springy loops after heating to incandescence became stiff and brittle.

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Richard Hull
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Re: Solderless Grid Design with Shaft Coupler

Post by Richard Hull » Fri Apr 17, 2020 3:47 am

Fixed Tungsten will embrittle rapidly in the d2 atmosphere and lose all its spring like tendency freezing it like brittle glass forever as any tension is relieved due to red heat cycling and embrittlement will follow rapidly in the hydrogen atmosphere. Stuff might work loose over time but re-tighten, if needed, very gently lest the W snap like an uncooked noodle. You might get some sparking off little sharp points at first runs so go easy.

Richard Hull
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Jim Kovalchick
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Re: Solderless Grid Design with Shaft Coupler

Post by Jim Kovalchick » Fri Apr 17, 2020 9:52 am

Years ago, my son used a set screw coupler for his grid. The set screw material was different than the barrel which was stainless. The screws spewed sparks and started evaporating. It plated out all over the inside of his chamber. I found some replacement stainless set screws for him, but he discovered that they had a soft metal (silver?) tip on them that evaporated. His grid ended up spinning during operation.

Dan Knapp
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Re: Solderless Grid Design with Shaft Coupler

Post by Dan Knapp » Fri Apr 17, 2020 12:08 pm

If you drill a depression in the feedthrough stalk where the setscrew tightens, it will stay on even if temperature cycling losses the setscrew.

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Nathan Marshall
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Re: Solderless Grid Design with Shaft Coupler

Post by Nathan Marshall » Sat Apr 18, 2020 8:44 pm

Thanks everyone for your comments and concerns! I got the whole assembly cleaned up with acetone and attached it to the feedthrough. I insulated the rest of the feedthrough stalk with an alumina tube. After pumping down, I put some voltage on the grid with my trusty rectified oil burner transformer. Richard, you were right: there was a little bit of sparking at first so I eased up on the power. But after a few seconds this subsided. At full power, the grid glowed on the side not connected to the coupler, but the opposite side attached to the coupler remained cool from the heat sinking effect. I ran it for a few minutes with the grid glowing and nothing came loose yet... I'll keep this thread posted if something does go wrong after multiple thermal cycles. Attached are some pictures of the grid operating at different pressures. Now on to higher voltages and powers once I get my newly constructed HV supply under oil.
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Rex Allers
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Re: Solderless Grid Design with Shaft Coupler

Post by Rex Allers » Sun Apr 19, 2020 7:41 am

Your work on this grid is really good, I congratulate you.

I'm thinking, though, that you seem to have a small cylindrical chamber. Maybe a cylindrical grid like one of Doug Coulter's designs might be appropriate to also try.

Here's one I made.
grid example.jpg

Needs a lathe to make, though. End pieces are turned from carbon rod. The straight "wires" are very thin tungsten welding electrodes.

There's been discussions about this grid in the past. If you have any interest in this I can look up links for you.
Rex Allers

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Bob Reite
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Re: Solderless Grid Design with Shaft Coupler

Post by Bob Reite » Mon Apr 20, 2020 10:18 pm

I found that the graphite end cap grids can take at least 500 watts input without trouble, while 400 watts was the maximum dissipation for the "traditional" wire loop grid.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.

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