Re: Pointseries gaps 'vs' other types
Subject: Re: Pointseries gaps 'vs' other types
From: Scott Myers <scotty-at-wesnet-dot-com>
Date: Wed, 14 Feb 1996 16:28:55 -0500
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> I am curious about material selection for a rotary spark gap. I too
> am interested in being careful with the rotor in the rotary gap. Do
> you know, or anyone else what a good material choice might be?? or
> phrased in another way did you select the lexan because of
> appropriate features or was it handy in the parts box? I was
> planning on ordering the plastic for the gap but need to resolve
> this point first. Do the metal parts get very hot--is temperature a
> concern for the selected material?
On rotary gaps, material choice is a BIG can of worms. Especially since you will have the
capability to drice it to 10,000 RPM, as do I. By considering this high speed, we are
breaking some new ground. This is an area where few have dared to tread, probably for
safety reasons. Lexan? If it were me, I would put it out of my mind. Lexan, even on a
small 8 inch disc will turn to shrapnel at 10,000 RPM. I am currently under construction
with my high-speed rotary too. I have spent considerable amount of time researching this
subject and talking with a few rotary gap experts, such as Richard Hull. I have also
spent much time talking to metalurgists, plastics manufacturers, high-speed balancing
companies, etc., ad neauseum. Whatever you do, take your time on this one. You are literally
building a bomb.
You need a material with a high tensile strength and little stretch. Phenolics are generally
about as good as it gets for an insulated rotor (for series use), unless you want to get really
expensive. G-10 phenolic (glass & epoxy) is a popular choice, although the linen & epoxy will
work as well. The G-10 is just considered the highest grade. If you build a hot rotor, then
an aluminum alloy is a good choice for the rotor.
The stresses to be encountered at a full 10,000 RPM will be tremendous, especially on a
large rotor. For quicker quenching, larger wheel diameters are needed. But the larger the
diameter, the greater the mechanical stress and the greater the need for dynamic balancing.
The pressures on a 10" disk's edges are incredible at 10,000 RPM. We are talking about several
thousand PSI, depending upon material choices. (Am I making you ill yet?) If you are
considering Classic coils only, then stay with a relatively small rotor diameter to limit the
mechanical stress. If you are thinking magnifiers, then quenching is critical and small
electrodes, fairly large rotors and high speeds are necessary.
As you get to speeds over 4,000 RPM, the balance will become critical. It will probably be
neccessay to get the assembly dynamically balanced for speeds in excess of 6,000 RPM. There
goes a couple hundred bucks (ouch). I personally got away (for the time being) from the idea of
a series gap because I wanted to run these high speeds and have a high current capability. I am
going to a multi point parallel aluminum rotor used in conjunction with a vacuum gap system.
Chuck, you asked if the points will get hot. It all depends on the amount of power you pump
through it and the speed you are turning. At high powers and low speeds, yes, the points will
get warm. At power levels below 4 KVA and speeds above 3000 RPM, I doubt if the points will get
hot enough to transfer the heat back to the rotor. But I would still build the rotor as though
they were going to get hot. There is no room for a melted rotor at 5000+ RPM!
I am building an safety housing around mine, in case of explosion. It will consist of layers of
sheet metal and automobile safety belts. Glenn Cerny put me on to this as this is a method used
by snow mobile racers around their clutch housings. I am told it works well for 10,000 RPM
This brings one question I would like to ask of others here to mind. Has anyone here ever
REALLY built a 10,000 RPM rotary gap. I will venture to guess that the answer is no.
> I've purchased flange blocks with bearings rated at 7500 RPM,
> (with reduced life above this) for my gap.
My torrington pillow block were also rated the same way. I called the manufacturer and told
them what I am doing. If you remove the grease and replace it with sythetic teflon impregnated
oil, 10,000 RPM should be no problem, since there is no side load.
> The Carter motor will
> drive through a Lovejoy flexible coupling into a 5/8" diameter
> centerless ground shaft. I've made an adapter to mate the "plastic
> rotor" to the shaft, next step is to order the plastic. There will
> be a minimum of 5 1/2" from the motor(motor case will be grounded)
> to any high voltage area on the gap--good or bad?
Plenty of distance. Shouldn't be a problem.