Re: Rotary Spark Gap.

To: "'Tesla List'" <tesla-at-poodle.pupman-dot-com>
Subject: Re: Rotary Spark Gap.
From: Tesla List <tesla-at-stic-dot-net>
Date: Fri, 5 Dec 1997 23:59:15 -0600
Approved: tesla-at-stic-dot-net

From: 	Bill Lemieux[SMTP:gomez-at-netherworld-dot-com]
Sent: 	Friday, December 05, 1997 4:22 PM
To: 	Tesla List
Subject: 	Re: Rotary Spark Gap.

Tesla List wrote:

> Don't use plexiglass for the rotor material.  The high temps can heat up
> the plastic and release your rotating electrodes.

Given the poor tensile strength of acrylic, it probably wouldn't have to
heat up before the material failed from hoop stress.

>Better choice of
> materials is type LE or CE phenolic.  1/2 inch thick x 11 in dia running on
> a standard 1/3 HP 1725 motor works fine.  Use approx 10 rotating electrodes
> and 4 fixed electrodes (4 total gaps) with two fixed electrodes on each
> side of the motor.  Tungsten/molydenum alloys work best

If someone is using W/Cu electrodes of any size (say 0.5" x 1.5"), the
stresses will be too high even at 1725 RPM for phenolic.

I did a stress analysis with a mechanical engineer on a 1/2" thick, 12"
diameter disk, using eight electrodes at a 4.75" radius, assuming pure
(worst case) tungsten electrodes 0.5"x1.5".  Our calculations gave
stress numbers that, while below the actual yield stress rating for high
grade LE, didn't allow for any safety factor.  No competent engineer
designs a high-kinetic energy device without a safety factor of at least
2!

We elected to use G-10 for the rotor and LE for the stators.  This gives
at least a 2.5 safety factor.

PS: if anyone is interested, I have a source for 70% Tungsten / 30%
rods, 1/2" x 8", but they aren't cheap.

-Gomez

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