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Re: ASRG & clarification of "dangerous design"
Original poster: "Scott Hanson by way of Terry Fritz <teslalist-at-qwest-dot-net>" <huil888-at-surfside-dot-net>
Terry B., Terry F., Dan McC., Steve C., Matt D., Malcolm, et al:
Thanks for your comments and replies to my original post; it's encouraging
that so many people are taking the time to formulate serious responses.
A few clarifications about my comments on the "propeller" rotary gap concept:
1. The baseline "conventional" RSG against which the propeller design was
compared was assumed to have a rotor disk made from either one of the
epoxy/fiberglass composite materials (G-10, Garolite, etc) or
linen-reinforced phenolic. These materials will NOT "shatter" and fragment
under any reasonable (3600 RPM maximum) rotary gap usage, even if a flying
electrode is shed or there is an impact between flying and stationary
electrodes. Any design that uses a rotor fabricated from an un-reinforced
material will be dangerous also, as acrylic ("plexiglass") or Lexan WILL
completely shatter under shock loads, especially if stress risers like
electrode holes, etc are present. However, the fragments from a burst rotor
will have less sectional density than a piece of tungsten rod, and will
have far less kinetic energy and penetrating power.
2. The word "makeshift" did not imply poor workmanship or stupidity, it
meant not having the correct tools available to do the job in the proper
manner. The implication is that this type of rotary gap could be quickly
built at home with nothing but a hack saw and a hand drill.
3. My comments were not meant to be "destructive" criticism; they were a
warning that this design would not meet conventional design or safety
standards for rotating machinery.
4. Terry B., your analysis of the actual forces involved is very useful to
help see the "theoretical" safety margins of your particular gap. Your
measured "slip" load of 20 lbs is also interesting. I believe that in the
US, many builders would use 1/8" tungsten rod, which is generally the most
commonly available diameter, rather than the 5/32" rod used in your
calculations. This would increase the "pull-out" loads acting on the rod as
a result of imbalance. Substitute 1/8" rod in your analysis, and the safety
factor is further reduced beyond the 2X factor due to the 1/2*L error in
5. I would challenge anyone who has made one of these gaps to run a simple
test: will the gap run continuously for 1 week without self destructing? I
don't mean commutating an operating Tesla coil, but just sitting alone in a
corner behind a shield with the motor running. This might seem extreme at
first, given the duty cycle of a Tesla coil, but certainly a rotary gap
could be expected to see several hours of cumulative run time over the life
of a coil. Demonstrating a survival time of 20X actual run time (or 40
hours) is not unreasonable, given the possible consequences of failure.
Again, my comments were meant to highlight the fact that this type of
rotary gap has some additional risk factors not present in conventional
disk-type rotors, and that suitable protection for any rotary gap needs to
be in place to deal with a worst-case failure. I have seen more rotary gaps
run without "scatter shields" than with, and everyone running a rotary
needs to consider shielding an integral part of the design, not an
accessory to be added "sometime later after everything is running OK".