FW: Rotary Gap

From: 	Edward J. Wingate[SMTP:ewing7-at-frontiernet-dot-net]
Reply To: 	ewing7-at-frontiernet-dot-net
Sent: 	Saturday, August 02, 1997 10:55 PM
To: 	Tesla List
Subject: 	Re: Rotary Gap

Tesla List wrote:
> From:   RODERICK MAXWELL[SMTP:tank-at-magnolia-dot-net]
> Reply To:       tank-at-magnolia-dot-net
> Sent:   Friday, August 01, 1997 11:25 PM
> To:     tesla-at-pupman-dot-com
> Subject:        Rotary Gap
> The present gap I,m using is a Rh type gap. But one of the members on
> the image list clued me in to the fact that I was not getting enough
> quenching. At first I thought to buy a shop vac monday and build a
> vacuum gap. But being the cheapskate that I am I started looking around
> the house for a cheap substitute! Here is what I found:
>       Two 1/3 horsepower motors.
>     One was in an old attic fan, and the other was in a old snowcone
> machine. Both work well. But the motor in the snowcone machine needed a
> new start capacitor. Since this motor is capacitor start and has a
> higher starting torque I,ll use it in the rotory gap. the other one I'll
> use as the drive for a coil winder. Both motors are westinghouse and
> have pulleys and belts attached. Both are also reversesible if that
> helps! The snowcone job has the option of running from 240 or 110vac.
> One interesting thing about the pulley on the snowcone motor is that it
> is adjustible. You can use wider or narrower belts by loosening a set
> screw and unscrewing half of it. This how I think I will mount the rotor
> by removing half of the pulley, putting the center piece thru the center
> hole, and remounting the other half. I also have some 1/4" black
> plexiglas. I wonder if I laminated two pieces together would it be
> strong enough? Any suggestions? Or should I use a stronger plastic? Help
> from any of the listmembers that has built a rotory gap in the past will
> be most appreciated!
>                                     Frankensteins Helper
>                                            Max


DO NOT, under any circumstances use acrylic plastic ( plexiglas ) for a
spark gap rotor. Acrylic cracks and shatters rather easily and is an
accident waiting for a place to happen when used as a rotary gap rotor.
Even small flaws from drilled or tapped holes will develop into cracks
as the material ages. Just the thought of a plexiglas rotor makes me

I build and sell rotary gaps and the only material I ever use is G-10
glass epoxy. G-11 has a little better heat resistance but is quite a bit
more expensive. IMHO there is no better material for rotors except for
steel or aluminum which leaves one with the added problem of insulating
the rotor from the motor. Lexan can be used, but melts at a much lower
temperature than G-10 and overheated rotor studs can work loose. I
regularly spin the 10" rotor on the series rotary gap I use on my
magnifier at 7600 RPM or better ( verified with an infrared strobe tach)
with no problems. A couple years ago while prototyping a 10" synchronous
gap, I crashed and dead stopped a G-10 rotor with 1/2" tungsten studs
from 3600 RPM on a 2 HP motor. The rotor was trashed because of cracks
around the stud holes but was otherwise intact. The stuff is just about
bullet proof!

So, if you want to do it safely, spring for the bucks for a piece of
G-10 or for a few bucks less a piece of Lexan, but please don't use

Safe Coiling,

Ed Wingate