Plastic screws

Am 17.07.19 um 15:25 schrieb Phil Hobbs:

M1 stainless steel + 2* TO-3 / TO-220 fiberglass-reinforced insulation washers?

cheers, Gerhard

On Jul 17, 2019, Phil Hobbs wrote (in article):

I think you will have to machine the non-nylon hardware. Delrin is a good

press-fit delrin, so threads may not be needed.

them in compression.

Or, if thermal insulation is the only rest, a stack of shiny steel plates with staggered metal separators (which can be spot-welded), so the leakage paths are very long, and radiation coupling between plates dominates (there been no air to support convection).

Joe Gwinn

Too big a heat leak. A larger plastic part with steel screws could work, but this gizmo has to fit in a really tight space.

Cheers

Phil Hobbs

Ceramic is a surprisingly decent thermal conductor, unfortunately--better than glass, and much better than plastic. SEMs don't run in UHV, so FR-4 is okay. At lower delta-T you can just have the board hang ten over the TEC and use the FR-4 for insulation.

Interesting idea about the delrin--maybe I could use unthreaded plastic studs with those speed-nut-like grip gizmos on the other end. (What are those things called, anyway?)

Cheers

Phil Hobbs

Got a sketch of the geometry?

Well a bunch of silly ideas... The Delrin sounds OK, maybe a machine shop can cut a thread on the end of a rod for you. (There are a few places on the web listing delrin screws, but no prices.)

How about soldering/ or epoxy and then squeeze it some other way. (Fishing line/ zip ties, something else?)

You need my beer can cooler on the hot plate :^) Has anyone done a distributed two stage TEC, Some small TEC in vacuum and then a bigger one outside the vacuum going to a heatsink?

George H.

On a sunny day (Wed, 17 Jul 2019 09:25:04 -0400) it happened Phil Hobbs wrote in :

Dunno, magnetic levitation?

Push nuts or self threading nuts. Also, mcmaster.com has fiberglass threaded rod but the smallest is 3/8"-16, for $15/foot so not completely hateful. They also have plain FRP fiberglass rod down to 1/8" diameter you could use with the push nuts. I think that would be stiffer, stronger, and less prone to creep than delrin.

PEEK is a pretty good engineering plastic that will tolerate hard vacuum fairly well provided that you don't want to bake it too hard. I don't think you are going to like their prices though. eg.

Or for a one off buy some PEEK rod and thread it yourself.

If the sizes work out, you can just scrunch a ss nut onto a Delrin rod.

Stainless steel spring? That wouldn't conduct much heat.

Don't base your pricing on McMaster. Their core business is the one-off, priced accordingly.

Buy the parts you need now, and source them [cheaply] later. (They'll tell you who made the part, on request.)

It really doesn't matter until you need actual boxes of, or pounds of, fasteners.

Tim

-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website:

I'm just curious about what you say of nylon in a vacuum. While plastics a bsorb moisture and that moisture will be released in a vacuum, the moisture should not strengthen the material, in fact moisture reduces the strength. I see they refer to moisture increasing the "toughness" of nylon which I believe means resistance to shock, so I guess pulling the moisture out make s it more brittle.

Polycarbonate is an affordable material. It seems to have better propertie s for your application than Nylon, although that's not a high bar. Here's one source.

How about stainless screws with nylon step washers as heat insulators. Especially in vacuum, there is no heat conduction without compliant thermal conductors or grease, only radiation. You'd be AMAZED at the poor thermal conductivity of flat, clean surfaces clamped together, in vacuum. Thermal conductivity drops by a factor of 100:1 when the air is pumped out.

Jon

Hi,

If you can get the torque accurate, low conductivity sufficiently strong wire through the same screw holes and twisted with pliers to tighten might be simplest.

cheers, Jamie

Amazon has acetal (delrin) screws for a few cents each.

Can you use rubber rivets?

They could also provide tension.

McMaster -Carr had threaded rod, but the smallest was

Back in grad school I made threaded rod with the right size rod, (maybe with help of a lathe) and a die. I once just ran a die down a piece of rod, (~1/8" Tellerium copper) by hand... the thread got all wiggley-woggley after a while. (I needed to hold the die perpendicular to the rod axis... I was much younger then.)

George H.

Oh Phil, I wanted to ask how many screws, you are using to hold down the TEC. (I've used 2 and 4, I can make arguments for 1 and 3.) Curious minds want to know, George :^)