Re: My first ARSG, or a Dangerous Design?

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

Original poster: "Crow Leader by way of Terry Fritz <teslalist-at-qwest-dot-net>" <tesla-at-lists.symmetric-dot-net>

 > Original poster: "Barton B. Anderson by way of Terry Fritz
<teslalist-at-qwest-dot-net>" <classictesla-at-netzero-dot-com>

[cut]

 > >with say a 4-40 screw on a 1/8" tungsten rod? You are going to get a
 > >stripped allen wrench before lots of pressure.
 >
 > Agreed, that's getting small.

The only "high speed" situation where I see set screws at 10,000 RPM would
be electric model airplane motors and the propeller mount being attached to
the motor shaft. This is using the collar and set screw the "normal" way
where it just keeps the shaft from slipping during rotation. The failure
mode is scew flies out and propeller stops spinning unless friction wedges
the shaft tighter into the hub. Nothing too bad, you should not be hanging
out near a propeller anyways.

 > > > is no "dig into" the tungsten, but there is definitely pressure. It's
nice
 > >
 > >This pressure is going to drop if anything slips. The the vibration will
go
 > >up, the collar slides towards the edges of the rod and the forces go up
even
 > >more. Any slippage makes it useless. Add the force of the opposing side
 > >collar that just creeped away from the center of the motor shaft. Now
it's
 > >working to pull the other side off. Unless you bite into the tungsten
rod,
 > >you are not locking things in place in a secure way.
 >
 > In the picture for this gap and in my particular disk, the velocity of
 > spinning electrodes is applying force of the electrode to the the disk.
The
 > screw is not required to retain the electrodes outer force away from the
 > motor shaft direction. The screw prevents electrode slippage from the
 > forces perpendicular to the outer rotational forces. In my case, the
 > electrode is centered to the disk and the set screw is centered to the
 > electrodes.
 >
 > Vibration is definitely an area of concern. My disk is well balanced and
 > vibration is minimum. Motor choice (wear, bearings, length between

Vibration deall makes things fly apart. I was working with a flyweel a littl
eover 1 foot in diameter on a 1/2" shaft of 1725 PRM electric motor. The
motor shaft had a flat and was soft enough for the screw to "bite" into.
Somehow this screw kepts coming loose and flying out, then the flywheel
would slide off the motor shaft and sping around in place while burning
marks into the floor before rolling away into something really hard. It was
fairly amusing. It was a small amount of vibration causing things to fall
apart. The size of the flyweel  vs. motor weight made sure the flywhel spun
"true" even thogh the bore for the motor shaft was a bit off. This made the
motor vibrate. Screws don't tend to fly out of collars in less violent 1725
RPM situations.

 > bearings, rotational speed, etc.) is important for minimum vibration as
 > well as disk balancing. However, not all of my gaps have been this well
 > tuned. I'm sure others may have some noticeable vibration in their gaps.
If
 > I had a gap with vibration, I would not use set screws (tungsten or other
 > any other material) regardless. Actually, I'd reduce the vibration. How
 > depends on the cause.
 >
 > > > for adjustments to keep the electrode "unscathed". I've used a set
screw
 > > > setup on a G10/tungsten disk for quite sometime now, and have never
had a
 > >
 > >Which way were the forces acting in this use?
 >
 > Ah yes, as mentioned above, perpendicular to rotation.

Ok.

 > > > set screw (1/4") or electrode  (.375" x 1.25") come loose.
 > >
 > >you don't generally use 1/4" set screws on 1/8" or 5/32" rods. It's not
hard
 > >to torque a 1/4" UNC bolt to several thousand pounds. The clamping force
of
 > >tiny set screws is way less.
 >
 > Agreed.
 >
 > > > Just don't use screw locking adhesives (Loctite, etc.). Electrodes get
 > >hot,
 > > > thus, set screws get hot. This heat causes adhesives to expand and
begin
 > >
 > >There are set screws with various plastics inserts wedged into a
lenghtwise
 > >slit in the threads. These add a decent amount of friction to keep a
loose
 > >screw from unscrewing itself.
 >
 > I would personally have fears of melting the plastic. I prefer metal to
 > metal contact.

I guess they are the screw/bolt equivalent to hex nuts with the nylon
insert, except nylon insert lock nuts make full contact with the whole
threaded part vs. "one slice of pie" in the case of a screw with a milled in
slot.

KEN