Re: Garolite (G9, G10, G11) questions.

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Sam,

Nice to hear from you again :-)

I use all these materials in battlebots where most of my testing is done in
ballistic materials for such things. >:o))

Avoid Lexan - It burns like oil, twists and flexes, does not like heat, and
can come apart dramatically.  It is far stronger than say Plexiglass and
polypropylene but it just is not right for big high speed gaps.  Many
people do use it, but it is not the "best".  Even Delrin will break if you
hit it hard enough, but Delrin IS my material of choice for "plastic stuff".

Avoid the "paper" phenolics (XX) since good phenolics are not much more
expensive.

Grade LE/CE Phenolic is basically linen/canvas in epoxy.  Used for years
since it is not terribly expensive and it is rock hard.  I have never heard
of it coming apart but it is the weakest (like Arnie's weak arm :o)).  I
don't use it because it has poor resistance to fire.  It burns right up in
fact.  It is about 1/2 the cost of the glass phenolics.

GPO3 - That red stuff that is used in everything electrical.  It has
amazing resistance to surface and electrical arcs but it is "soft" and the
fibers in it are random.  Great electrically but I worry about the
strength.  It is glass fiber in polyester resin?  Not good for a gap but
great for other electrical insulating needs.

G10/FR4 is the common PC board material used in quality electronics.  Rock
hard and impervious to physical damage.  Flame proof, projectile proof,
chemical proof...  Woven glass fiber in epoxy.  It "can" be damaged but
everything else on the gap will disintegrate first.  Not too expensive
since it is so common and easy to find.  The material of choice for my
gaps.  You can burn it up but it is too hot then anyway.

G11/FR5 - The new and improved version of G-10 with better chemistries.  I
don't think one would notice any difference over G-10 except for a
remarkable jump in the price.

G5/G9 G-wizz :o))  Another version of glass epoxy.  it is supposed to be
stronger but I don't think it matters.  Only a little more costly than
G-10.  Melamine resin...  Might be fun to try.

There is also a G-7.  Silicone is used instead of epoxy.  Great electrical
and heat properties but I am not sure how strong it is.  This price is
shocking...  I don't know much about this one but it looks interesting...

I would avoid Teflons since they burn with wildly toxic gasses, cold flow,
expensive...  The glass filled is used at work now but I would not use it
for a gap.  Too weak really.

There are "super phenolics" with like woven Kevlar, carbon fiber, arimid
(sp?) fiber... in them but the cost is not worth it unless you need a gap
than can go to war (with a similar budget and suppliers).  I think these
are known as military or ballistic grades.  Probably have to answer lots of
questions to buy them.  I think they are export controlled and all that...
If you are feeling lucky, I guess you can make them yourself for a fraction
of the cost.  "You" would probably have the skills and knowledge to do this ;-)

So "i" would just get good o'l G-10.  If you want to get a little exotic
maybe G5/G9.  I think you would waste too much money on G11/FR5.  Really,
these materials are far stronger than you need and you will not have to
worry about them.  They are still much better than the canvas versions.
The glass filled stuff at greater than 3/8 inch will break the motor,
mounts, and electrodes before it breaks...  I have never known "thick" G-10
to delaminate unless you take a torch to it.  A high powered rifle round
"might" go through it but shattering is not an option.

Of course, all the glass phenolics will eat tools like crazy!  Cut the disk
on a band saw with cheap blade you plan to discard.  Any cheap blade will
dull just as fast as an expensive one here.  Table saws with carbide blades
eat through it and survive for a one time thing just fine.  TiN drill bits
or even cobalt bit$ will drill the holes and survive too.  Don't even think
about milling it.  You don't need to anyway.  I just mount the disk in the
motor and use carbide sand paper and a hard surface to true up the circle.
Sort of a power sander in reverse.  Better than a lathe IMHO.

The dust is nasty like finely ground fiberglass insulation.  Get a good
mask, gloves, and wash/clean up good afterward.  It never bothered me, but
check to be sure there is not anyone around that will not like such dust
since it is pretty irritating to some folks.


Cheers,

	Terry




At 03:25 PM 6/8/2002 -0400, you wrote:
> Greetings,
>
> This question concerns materials choice for a rotary spark gap. 
>Obviously we want the rotor disk to be as light as possible so that 
>a smaller motor can be utilized and the spin up time becomes shorter.
>In order for a light disk to withstand the centrifugal forces encountered 
>in rotary spark gap duty it would make sense to use the strongest 
>material possible (within some price restrictions obviously). I notice 
>that G-10 grade Garolite seems to be the most often used material,
>and with a tensile strenght of 40000PSI and an impact strenght of 
>7ft/lbs/in it is definitely a good choice. I also see some designs 
>using polycarbonate but at 9000PSI tensile strenght, 12ft/lbs/in 
>impact strenght I would definitely go for G-10.
> However, why doesn't anyone use grade G-9 Garolite? With a tensile 
>strength of 66,700PSI lenghtwise and 51,900PSI crosswise, and an 
>impact strength of 14.5ft/lbs lenghtwise and 11,2 crosswise, it would 
>make a much better material choice for a lighter, stronger rotary 
>spark gap disk, no? The price is also virtually the same as G-10 
>($29 for a 1/4in thick, 1'x1' sheet). Also, what about other grades 
>of Garolite (G-11, G-30)? They seem to be weaker and more expensive,
>is there any advantage to them? Is there any plastic/composite material 
>that is NON CONDUCTIVE and stronger than G-9?
> Finally, how stiff is G-9? Does it buckle under force or does it 
>tend to fracture in a brittle fashion (I am deducing from the relatively 
>low impact strength that it will shatter, but I couldn't find its 
>modulus of elasticity anywhere)?
> Any answers to these questions would be greatly appreciated.
>
>Sam Barros
>ME/EE Major at Michigan Tech.
>http://www.powerlabs-dot-org/
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