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

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

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-