Re: Rotary Gap

Scott Myers wrote:
> >I have not
> >tried to do the calculations on aerodynamic drag on the electrodes, but it
> >has to be
> >considerable.  There is also the bearing drag to consider.  My motor is a
> >full 1 HP unit and I am
> >wondering if I will have enough hp to ever reach 10000 rpm's.  If it
> >doesn't, I'll have to buy a
> >second motor to add to the drive.
> >
> >Another thing to consider is the need for robustness at these speeds.  My
> >wheel will be an
> >aluminum/magnesium alloy.  It will be dynamically balanced.  The shaft
> >will be of through
> >hardened tool steel.  That is what I felt I needed to turn at that speed
> >safely.  Bottom line:
> >It's a lot of mass to be turned very high rate of speed.  Even with a
> >dynamic balance, there will
> >be some vibrational losses.  I don't think I would get there easily with a
> >small hp motor.
> >
> >1/2 hp?  Seems a bit small to me, at least for the size of a disk I am
> >constructing and the
> >sppeds it will turn.  For a smaller disk, it is probably just fine.
> >
> >Scott Myers

Jerry Biehler wrote:
> I am curious, what kind of mechanical experience do you have? I dont want
> to sound overly critical, but your plans for a rotary spark gap are over
> kill so say the least.

I have 15 years working as a technician/salesman in the hydraulics industry.  Without going into 
detail, let's just say I have been around the block a few times when it comes to mechanical 
construction.  I have explored about every angle of this rotary that you can imagine.  This 
includes both safety and performance.  This rotary represents a good mix of both.

Let's see.  Overkill?  My intended maximum speed is 10000 rpm with a 12" rotating disk.   At 
10000 rpm's, the edge of the wheel will experience a 3700 psi centrifical (tensile) pull and a
4500 psi radial stretch, according to a metalurgist.  While this material is capable of more than 
 8 times those stresses unmachined, it is a different story when I machine threaded holes into 
the edge.  This introduces stress risers that will greatly degrade the mechanical integrity of 
the edge of the wheel.  No one can tell me by how much.  I is a matter of experimentation.  The 
metalurgist agrees that it shouldn't be more than 50%.  Let's say it does degrade it by 50%.  
That leaves me a 4:1 safety margin.  That is an industry minimum, for safety reasons.  If 
anything happens to go out of balance while at full speed, I am going to need every bit of that 
margin to keep it fron flying apart.  There will be a safety shroud around the wheel just as a 

I have seen machines in the hydraulics industry that were built with a 10:1 safety margin fly 
apart, probably due to a mechanical defect.  I have seen good people die because of their (or 
someone else's) inexperience and/or impatience.  A high speed rotary is a potentially lethal 
device.  If it blows apart and you are in the plane of the wheel without a bell housing, you are 
going to be hurt, if not killed.  There is no way you can build one with a safety "overkill".  If 
I could, I would build it with a 20:1 safety margin.  In my mind, safety is first and formost.  
Everyone know's the dangers involved in this hobby that does it.  We all accept that and deal 
with it.  I personally have a wife and 3 kids.  I must take safety VERY seriously.  If not for my 
sake, than for theirs.  I don't want to leave my wife a widow and an kids without a father.  I 
know this sounds very forboding, but it's the real world.  This is uncharted territory for me and 
I can't afford a mistake.  Call me paranoid if you wish.

If anyone here decides to build one with a relatively large diameter and a high rotational speed, 
keep the above in mind.  Do your research.  Study it VERY carefully.  Don't rush in blindly.

> As for a motor. I dont think you realize how much power 1 HP is. A half HP
> motor would easily get up to 10000 RPM. In fact that may still be overkill.
> This is oviously a brush type motor, the speed stamped on the side of the
> motor is an approximation. These motors were never made to run at exact
> speeds. The best way to regulate the speed on a brush motor without using
> costly PWM and encoders (Not to mention, sensitive to RF) is to use a
> centrifugal switch that kicks out at a certian RPM and kicks back in just
> below the desired RPM. This happens so fast all you see is a blue ring
> where the contacts are closing and opening. If you cant get up to 10000
> RPM, use a variac to give it a little more voltage. The motor can take it.

I intend to use a variac with a 0-140 Volt output.

> A small motor may take a bit longer to get up to speed but once it is
> there, there is no real load.

Not even an aerodynamic load?  Have you ever stuck you finger out of the window of a moving car 
as a kid?  I'm sure you have.  Now here is a car that is moving 416 mph and has 12, 1" long 
"fingers" sticking out its edge.  Granted that the previous electrode is breaking the air for the 
next one, to some degree.  I just can't believe that there is going to be no load.  I would love 
to hear someone else's experience with electrodes moving at this velocity, if there is anyone who 
has been there.  For those that are curious, the electrodes are #10-NF threaded with a carbide 
tip.  They are ground down near the tip so it is a 1/8" diameter at the very tip.

> As for the materials you are making the wheel out of, WHY? There is no need
> for Aluminum-Magnesium alloys. Steel would probaly be a bad choice, but
> stainless would be fine. In fact, if you used a denser material, the wheel
> would act as a flywheel which would smooth out RPM changes due to power
> fluctuations. I agree, dynamic balancing is a must.

Aluminum is the BEST choice, in a world of trade-offs.  This is a belt driven "hot" rotor.  I 
want the aluminum for its conductive qualities.  Q is everything in the tank circuit.  Stainless' 
conductive values are nothing that I would be happy with.  The electricity will commutated from 
one side of the wheel to the other.

As far as the flywheel capabilities, it will weigh plenty.  It is 12" Diameter by 1.5" thick.  It 
should weigh at least 10 lbs.

Another thing I didn't mention.  I am eventually going to try to turn it 20000 rpm's.  I will 
never make it as the rotor tip speed would break the sound barrier.  I am going to find out where 
the mechanical limit is, and how much hp is required.  Another reason I need a very robust wheel.

> Another option is you could use a fixed speed AC induction motor. Since
> they come in standard speeds (1725, 3450, ect)that are regulated by the
> input frequency. You would have a gap that would have almost no
> fluctuations. If you used a belt to drive the spark gap it would also
> insulate the motor from the tank circut a little better.

For my intended purposes (eventual magnifier work), these motors are too slow.  There quenching 
would suffer.  I am looking for quench times to the tune of 30 microseconds.  This short 
dwell duration is the very thing that has driven me to a 14" electrode wheel diameter, 1/8" 
electrode tips and the high rotational rate.  Fixed speeds are also not an option.  The motor 
must be variable, so I can vary the break rate.

True that a variable frequncy drive would be a good thing to use with one of these motors.  It 
would be nice too.  I could use single phase to drive the amplifier which would produce 3 phase ( 
A Baldor variable frequency drive does this).  It would give me many advantages, exact speed 
control with readout, dynamic braking, controlled accel and decel rates, etc.  There are 2 
reasons I can't use this method.  1. RF interference.  I would certainly lose control if I could 
not sheild the cables and controller well enough.  2. Price.  I can get these units at 
manufacturer's cost, but it is still painfully expensive.  It would also require that I have a 
motor armature dynamically balanced, as a standard motor would not take it.

I figure that if I want to get a speed readout, I can eventually use a DC tach generator with 
current output, for noise rejection.  But befort I get there, I need to finish the rotary first.

> Oh, btw, we are getting a metal lathe real soon, I probaly will be able to
> make gap wheels if people need them.

Careful here Jerry.  Better make sure you have folks sign a non-liability agreement, if you make 
one for them.  You never know what someone else is going to do to a piece you built.  They could 
turn it to 40000 rpm's for all you know.

I'm taking off for the weekend.  I'll answer any responses to the above after I return.

Scott Myers