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Re: [TCML] Rotary Disc Material

Andrew -

Any polyethylene material is completely unsuitable for use as rotor material in a Tesla coil rotary spark gap. Polyethylene (even UHMW material) is extremely soft and has very low tensile and shear strength. Even worse, the heat-deflection temperature is extremely low, so the material will begin to distort at very low loads at even moderate temperatures.

An RSG disk mounted on a high-speed motor is subjected to very high mechanical loads, and needs to be carefully designed and constructed so as not to be a potential bomb. The  structural failure of an RSG disk at full speed can release a tremendous amount of energy, and electrodes can "leave the scene" at very high velocity.

There are several materials that are much better candidates than PE for RSG rotor material, but they still have temperature limits and stress limits that must not be exceeded:

1. Fiberglass fabric reinforced epoxy laminate (Grades G10 or G11);
2. Fiberglass fabric reinforced melamine laminate (Grades G5 or G9);
3. Canvas or linen fabric reinforced phenolic laminate (Grades CE or LE).

The fiberglass-reinforced materials are strongest, and have the best high-temperature characteristics, but are difficult to machine and require carbide or diamond cutting tools to get decent surface finishes and tool life. All of these materials have good electrical characteristics (resistance to arc tracking, etc)

The phenolic laminates are much easier to machine, and work well IF the electrode temperature can be maintained within the material's temperature limits. If the temperature limits are exceeded, the resin binder will begin to decompose, causing loss of strength of the material, loss of dimensional stability, shift in electrode position, etc.

The G9 and G10 materials are good for ~285 deg. F; G11 is good for ~300 deg. F, while the phenolic-based materials are good for ~265 deg. F.

Herr Zapp

--- On Wed, 4/22/09, Andrew Robinson <teslamad@xxxxxxx> wrote:
From: Andrew Robinson <teslamad@xxxxxxx>
Subject: [TCML] Rotary Disc Material
To: tesla@xxxxxxxxxx
Date: Wednesday, April 22, 2009, 4:54 AM

Check out this image of my old SRSG:


The disc used UHMWPE (Ultra high molecular weight polyethylene) as its base
plastic. Ignoring the burned areas, what im really concerned about is the
melting... I know all this looks bad, bear with with me a min.

(Side note: UHMWPE has a melting point around 144 to 152 degrees C)_

This gap ran a 10KVA pig driven coil hence the crazyness. Those steel
electrodes got so hot that they melted into the plastic. The washers that used
to be there are now completely gone... vanished... MIA. lol

Anywaz, the new gap is going to use tungsten electrodes that space out farther
from the disc to prevent the burning , the the new electrode material should
help with the heating up problem, but as an extra measure, im looking for a new
material to machine the disc out of. I've seen alot of people out there that
made the disc out of what looks like a brown brittle material. What can I use
that will chock it up to the heat and be an excellent insulator?

Andrew Robinson
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