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Rotary SG Safety

Original poster: Zimtesla-at-aol-dot-com 

Tesla List readers,
Recently a friend of mine who has been building Tesla coil projects for 
over 20 yrs, built a small asychronous rotary gap. He wanted it for use on 
small coils so it has only a 6" rotor disk. He used a small universal motor 
from a vacuum cleaner. The rotor is 6" by 1/8" Lexan and the rotating 
hardware was #8 brass acorns and all thread.

The rotor and motor were fully enclosed in plastic. The rotor case was 1/4" 
Lexan. The motor case was acrylic. The overall appearance is like an office 
fan but made of plastic. The finished project was a work of art with 
everything carefully balanced so that it ran with no noticable vibration.

When I was over to see the progress on the rotary,  he had not installed 
the stationary electrodes which were to be tungsten. We spun up the unit 
and made speed measurements with a strobe tach. At 60 VAC the little motor 
was turning over 6000 rpm and the unit had somewhat of a siren quality to 
it. We stopped at 70 VAC as this speed was more than would be needed. I am 
always a little wary when near rotating equipment that is in the whine to 
siren mode.

A few days later, my friend was not in a great mood. He had installed the 
stationary electrodes and gapped them at around 0.040". He decided to run 
more safety checks and cranked it up to 80 VAC. At this point there was a 
loud bang and shrapnel proceeded to eject through the 1/4" Lexan and some 
small threaded parts left an indent in the shelves across the room. He 
stated there were parts 30 ft from ground zero. Fortunately nothing hit 
him. The probable cause was that the 1/8" rotor flexed enough due to air 
pressure being ported through special holes for that purpose so that it 
caused the flying electrodes to be sheared off. The Lexan disk did not break.

Fast forward. Due to his experience, I decided to rework my tungsten rod 
propeller gap as used by some folks on this bulletin board. I had a 4" tall 
fiberglass shield around it but it was fairly flimsy. So I replaced it with 
a "firewall" of 1/4" masonite I had around. The masonite has bricks 
embossed on one side - thus the term firewall.  The shield was much deeper 
(rotary is horizontal) so that flying parts have to bounce around a 
while  before leaving the area. Today when I was running this rotary, I 
heard a moderate "bang". The system shutdown on its own so I figured on a 
blown cap. Instead, the tungsten propeller rod had turned into three pieces 
and hit the firewall without too much ado. I was glad for the added safety 
of the masonite.

I will forward photos of the Lexan rotary to Terry  for possible inclusion 
in his hotstreamer site.

Keep your coiling safe. Assume your rotary will fail so provide shielding.