I made aluminum collars for my electrodes with 1/2" aluminum bolts (then
drilled a 1/4" hole down the axis of the bolt. Then I tapped an 8-32
thread
for a set screw to hold the electrode in place)
One thing I did notice was that the extra material poking out from the 12"
disc (the collars) did create enough air drag that my SRSG (with two flats
machined into the rotor for 3600 RPM) would jump out of phase regularly. I
finally machined the collars so no extra material was exposed, and then
removed two of the contacts that would be passing through zero volts (it
now
has 6 electrodes instead of 8). This reduced the drag to a point that the
SRSG maintained synchronization.
Un-Terry
-----Original Message-----
From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On Behalf
Of David Rieben
Sent: Monday, June 16, 2008 8:47 PM
To: Tesla Coil Mailing List
Subject: Re: [TCML] G-10 blistering
Hi Phil,
I like the clamp-on shaft collar idea and went ahead and ordered 16
of those aluminum ones from McM-C today. As long as I place one on
each end of each of the 8 flying electrodes, I don't see why the rotary
disc would need to be (re)balanced, so long as it was decently ba-
lanced in the first place and the motor is suffciently strong enough to
spin the additional mass of the added collars. This should definitely in-
crease the thermal mass and the aluminum would also conduct heat
away from the tungsten "core" electrodes better.
David Rieben
----- Original Message -----
From: <FIFTYGUY@xxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Sunday, June 15, 2008 7:27 AM
Subject: Re: [TCML] G-10 blistering
In a message dated 6/14/08 2:00:43 P.M. Eastern Daylight Time,
bartb@xxxxxxxxxxxxxxxx writes:
I have 8 flying electrodes on my disc. Each of these flying
electrodes is a 3/8" x 1.5" long solid tungsten rod.
Some notes with bare tungsten press-fit to G-10:
When I assembled the 3/8" tungsten rods into my 12" G10 disk (sound
familiar?) I did some testing in a scrap piece of G10 (the corners of
the
squares
I cut off to make a circle from the square piece of 3/4" thick G10). I
drilled a hole with the same distance-to-edge as the actual rotor holes
and reamed
it out to .373" to begin with. The rods were .375" dead-nuts, to the
best
of
my measuring ability. I had a .374" reamer in case they ended up too
tight.
It took about two tons of force to get the rods into the holes, or to
adjust
them once they were in. So I reamed all 8 rotor holes to .373".
Another thing I wanted to check was the retaining ability of G10 at
elevated temperatures. So with a torch and a non-contact thermometer I
carefully
warmed a rod up to 300 deg F (past the 284 deg F max service temp of
G10!).
Guess what? That press-fit rod now freely slid back and forth in its
hole!
After it had cooled, it was nowhere near a tight fit as it had been.
There has been discussion here that adding a retaining setscrew thru
the
rotor edge actually weakens the G10. Maybe the best bet is clamping
shaft
collars:
From McMaster-Carr (_www.mcmaster.com_ (http://www.mcmaster.com) ):
6157K13
One-Piece Aluminum Clamp-on Shaft Collar 3/8" Bore, 7/8" Outside
Diameter,
3/8" Width
In stock at $2.25 Each
or if you prefer thermal mass over conductivity:
6435K13
One-Piece Clamp-on Shaft Collar Black-Oxide Steel, 3/8" Bore, 7/8" OD,
3/8"
Width
In stock at $1.99 Each
The plain steel would probably suffer in that environment. They also
offer stainless steel for substantially more cost and lower thermal
conductivity.
I've seen some folks use shaft collars as the sole means of retention
on
their RSG's. Of course you'd have to re-balance after adding the collars
to
both sides of the rods. Would add about $40 for my 8-electrode RSG, but
certainly seems worth it if it keeps the tungsten slugs from escaping!
Might have
to watch out that the collars don't arc to the stationary electrodes.
Some folks use rings on the faces of the disk, but I dunno how they
secure the rods. I also dunno how you would get heatsinking from it
unless you
tack-welded the collars to the disk after everything was assembled.
Maybe the best would be to use a solid aluminum rotor with an
insulated
hub or belt drive. I've got all the parts for a timing belt drive, but I
figured I'd start simple.
-Phil LaBudde
Center for the Advanced Study of Ballistic Improbabilities
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