Re: ATM 'Frozen' liquid mirrors

[Suzanne Sarlette/Gerald Pearson <suegerry@mut1.muscanet.com>]

Some more thoughts about air bearings.  

The more precise and smooth the surfaces are, the thinner can be the
film of air that separates them.  The thinner the film, the smaller the
air flow which is required.  I suspect that, for conical surfaces in
particular, a thinner film also means less "wobble".  

I would speculate that the optimum thickness for the air film would be a
few times larger than the surface roughness/irregularity, and that this
would be achieved when the air-flow rate is a few times as large as the
absolute minimum required to "float" the load on the bearing.  One way
to achieve this might be to set the regulated air pressure at perhaps
1/3 to 2/3 atmosphere, adjust the air flow with a needle valve (located
between the pressure regulator and the air bearing) until the bearing
just barely "floats" the load, and then crank up the regulated air
pressure to maybe 2 to 4 times its starting value.  
        OTOH, perhaps a much higher air flow rate would be beneficial, because
of the Bernoulli (sp?) effect that tries to pull together 2 surfaces
when air is flowing between them.  I would imagine that this might
either stabilize things to reduce wobble, or it might turn out to
interact with the "pushing" of the compressed air in some way to
introduce some oscillation which could introduce _worse_ wobble. 
Anyhow, once things were set up, it would be easy enough to try out
different air-flow rates to see what gives the least vibration and
wobble.  The only difficulty might be that the vibration/wobble could
turn out to be so small that it might be difficult or impossible to
measure or even detect it.  I'm sure that the vibration and wobble would
be _much_ less with an air bearing than it would be with a conventional
bearing.

I suspect that it would also help to balance the load as well as
possible -- again, to minimize any residual wobble.

FWIW:

1.      The read/write head of a computer disk drive "flies" on a film of air
a small fraction of a thousandth of an inch thick, which keeps the head
from actually touching the surface of the "platter" -- the surface of
the disk.  A "head crash" descibes the situation where the head actually
touches the disk surface for some reason or another.  After a head
crash, it's usually necessary to replace the disk drive, or at least the
heads and platters.  A smoke particle is bigger in diameter than the
thickness of the air film, and a smoke particle on the surface of a
platter can cause a head crash.

2.      High-speed dental drills use air bearings.  That's how they can
achieve such high speeds (~50,000 revolutions/minute) without the
bearing burning up.

3.      In physics teaching situations, an "air table" is sometimes used to
demonstrate "friction-free" motion and collisions of "pucks".  The table
surface has a bunch of small holes drilled into it, which admit
compressed air from the chamber below the table surface.  The pucks are
small disks with diameters larger than the spacing of the holes in the
table.  At any time, a puck is covering several holes.  The air film
between the puck and the table allows for nearly friction-free
"sliding".

        -- Gerry