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

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

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jim-at-jlproduction-dot-com>

I would like to pipe in a dose of opinion here.
It is my uneducated opinion that you are all WAY too worked up about
strength and speed here. 1800 to 3600 rpm is small potatoes speed wise
and in a rotary gap use has no real load at all. I work in an automotive
machine shop and most abrasives which we use are only glue and grit turn
up to 10,000 rpm all day long. Add to that some severe plunge and side
load, dry cutting, extreme heat build up and general abuse and I have
never seen one come apart in nearly 15 years. You car and your house are
full of cheap junk materials that spin twice as fast and runs constantly
for years with no trouble. As long as its semi balanced and non
conductive I would think you could use practically anything for a wheel
and never have a lick of trouble.
You may want to, for example, look into the chop saw wheel like I used
as it has many of these properties and are dirt cheap. They can spin in
excess of 10,000 rpm, are thin and light, well balanced, can take
tremendous heat, machine nicely and have a variety of hole sizes from
the factory. They also are non conductive as far as I know.

Just my .02
Jim L
http://www.jlproduction-dot-com/Tesla.html


-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com] 
Sent: Saturday, June 08, 2002 8:11 PM
To: tesla-at-pupman-dot-com
Subject: Re: Garolite (G9, G10, G11) questions.

Original poster: "Dave Larkin by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <teslaman15-at-hotmail-dot-com>

>  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

The material needs to be adequately strong, 200% is a decent engineering

safety margin, 'as strong as possible' is rather unnecessary.

>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

G-9 is melamine based, G-10 is epoxy based, so G-10 has marginally
better 
high temp. characteristics.  G-10 is also a bit cheaper from my
supplier.

>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?

G-30 has _far_ better high temp. characteristics, safe to 500°F, as
opposed 
to 285°F for G-9,10,11

>  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)?

All glass laminates have quite a similar failiure mode - they bend quite
a 
lot, then shatter into fairly large chucks.

FWIW - Glass laminates are a pain to machine, and the dust is not good
if 
you value your lungs.  I have always used high grade cotton phenolic 
laminates and had zero rotor failiures.  Phenolic laminates have better
over 
temperature characteristics than epoxy based laminates (epoxy is 
_thermosoftening_...), and I'd bet the most likely mode of rotor
failiure in 
most systems is overheating.  Bill Wysock, Robert Stephens and Richard
Hull 
seem to agree...  Of course for really high power stuff the Aluminium
rotor 
can't be beat for heat dissipation, but you have to machine a very well 
isolated hub and are limited to only two gaps per wheel.

-Dave-