Aluminum composite reportedly stronger, lighter than carbon

On Nov 1, 12:21 pm, "Maxwell" wrote:
"Mark Hickey" wrote in message

...



Fred the Red Shirt wrote:

How big is his vacuum chamber?

What sort of vacuum pump(s) does he use?

One large enough for a 17.5" mirror is rather non-trivial. Assuming
a 20" diameter cylindrical chamber, the top and bottom would need
to support over 3000 pounds each, if the work is done at sea level.

Is it really that hard to build a vacuum chamber? Seems to me that
the most pressure it'll ever experience is about 15psi (1 bar), while
it's trivial to build/buy pressure containers that can handle 10-100x
that much (positive) pressure. Certainly if building a 1 bar vessel
20" in diameter is daunting, building a submarine (or worse, a
deep-sea bathyscaphe, which have reached depths of almost 36,000 feet
below sea level, resisting a pressure of about 1,100 bar) would be
unthinkable.

Or am I missing something?

Maybe, maybe not. Round pressure vessels keep their shell walls in tension,
hence the more pressure the better they hold their shape. Vacuum vessels are
just the opposite, and quite often much easier to collapse than one might
naturally assume.

I can say I once built a round vacuum chamber out of rolled 1/4" aluminum.
It was approximately 18" long and 18" in diameter. The bottom was 3/8"
aluminum, and the top was 1" clear plastic. The chamber was successful with
up to an near perfect vacuum, and used many times without failure. At
maximum vacuum, the bottom would dish approximately 1/8 to 3/16" inch, an
the plastic top would dish about 1/2".

I have a chamber I use now for another purpose, but it is only 6" in
diameter. The top for it is just 3/16" tempered glass.

Hope the number might help your estimates.


Ok so at 18" diameter that clear plastic dish had a surface area
of about 254 square inches, so it saw a force of about 3700 lbs,
less if you were significantly above sea level.

--

FF

On Nov 1, 8:53 am, Mark Hickey wrote:
Fred the Red Shirt wrote:

How big is his vacuum chamber?

What sort of vacuum pump(s) does he use?

One large enough for a 17.5" mirror is rather non-trivial. Assuming
a 20" diameter cylindrical chamber, the top and bottom would need
to support over 3000 pounds each, if the work is done at sea level.

Is it really that hard to build a vacuum chamber?

No, but it is much easier to build a small one than a large one.

Seems to me that
the most pressure it'll ever experience is about 15psi (1 bar), while
it's trivial to build/buy pressure containers that can handle 10-100x
that much (positive) pressure. Certainly if building a 1 bar vessel
20" in diameter is daunting, building a submarine (or worse, a
deep-sea bathyscaphe, which have reached depths of almost 36,000 feet
below sea level, resisting a pressure of about 1,100 bar) would be
unthinkable.

Or am I missing something?

Buckling.

The skin of a pressure vessel is in almost pure tension, so they can
be thin and not buckle. Any bending moment on a flat section bows
it outward reducing the bending moment (essentially converting it to
tension) The sides of vacuum vessel see compression and bending,
and any flat sections will buckle inward which will increase the
bending
moment.

The bathyscape and similar vessels are cylindrical with hemi-spherical
ends so that their skin is in almost pure compression with very little
bending moment.

A 55-gallon drum can be cut down to make the sides of the vacuum
chamber but I'm not clear on how to make the end hemispherical.
An option is to use nested vessels, with partial vacuum between them.

There is a reason why bell jars have a hemispherical top, and it is
not esthetics.

--

FF

"Fred the Red Shirt" wrote in message
oups.com...

Ok so at 18" diameter that clear plastic dish had a surface area
of about 254 square inches, so it saw a force of about 3700 lbs,
less if you were significantly above sea level.


Yes.

Another way to consider it, is that the 18" diameter, 1/4" wall aluminum
tube has to support the approximate weight of a small SUV.

On Wed, 31 Oct 2007 20:32:08 -0700, Fred the Red Shirt
wrote:

On Oct 26, 2:04 pm, "Roger (K8RI)" wrote:
On Fri, 26 Oct 2007 09:53:04 -0700, Fred the Red Shirt
...
Surface Preparation (cleaning) is extremely important as traces
or organics (from handling( will prevent adhesion of the aluminum.

Texereau recommends final cleaning by electron bombardment
in the vacuum chamber.

Which reminded me, my friend uses that plasma for the final cleaning.
Thats a lot of electronic bombardment. :-))



How big is his vacuum chamber?

What sort of vacuum pump(s) does he use?

The roughing pump is a two stage, dual vane, rotary pump with IIRC a
2.6 Liter capacity. (It's big and heavy any way and uses a 1 HP motor)

The diffusion pump is a 10" Siemens with a 3 stage chimney, but no
cold trap. The cold trap is there but only serves as a baffle without
cooling and appears to stop back gassing. It uses about 100 ml of 704
or 705 fluid (although the pump works well with anything between 50
and 150 ml).


One large enough for a 17.5" mirror is rather non-trivial. Assuming
a 20" diameter cylindrical chamber, the top and bottom would need
to support over 3000 pounds each, if the work is done at sea level.

He has a quartz bell jar that is (again IIRC) about 18"in diameter and
close to 18" tall or a bit more. Ultimate vacuum is probably on the
order of 1.2 or 1.3 X 10^-6 or -7 Torr although it doesn't need to go
that high.

Roger (K8RI)

On Thu, 1 Nov 2007 12:21:03 -0500, "Maxwell"
wrote:


"Mark Hickey" wrote in message
.. .
Fred the Red Shirt wrote:

How big is his vacuum chamber?

What sort of vacuum pump(s) does he use?

One large enough for a 17.5" mirror is rather non-trivial. Assuming
a 20" diameter cylindrical chamber, the top and bottom would need
to support over 3000 pounds each, if the work is done at sea level.

Is it really that hard to build a vacuum chamber? Seems to me that
the most pressure it'll ever experience is about 15psi (1 bar), while
it's trivial to build/buy pressure containers that can handle 10-100x
that much (positive) pressure. Certainly if building a 1 bar vessel
20" in diameter is daunting, building a submarine (or worse, a
deep-sea bathyscaphe, which have reached depths of almost 36,000 feet
below sea level, resisting a pressure of about 1,100 bar) would be
unthinkable.

Or am I missing something?


Maybe, maybe not. Round pressure vessels keep their shell walls in tension,
hence the more pressure the better they hold their shape. Vacuum vessels are
just the opposite, and quite often much easier to collapse than one might
naturally assume.

I can say I once built a round vacuum chamber out of rolled 1/4" aluminum.
It was approximately 18" long and 18" in diameter. The bottom was 3/8"
aluminum, and the top was 1" clear plastic. The chamber was successful with
up to an near perfect vacuum, and used many times without failure. At
maximum vacuum, the bottom would dish approximately 1/8 to 3/16" inch, an
the plastic top would dish about 1/2".

We used to use this on glass plates to grind the corrector for a
schmidt casagrain. Warp the glass with the vacuum, grind to a
parabola, and then release the vacuum. In this case it took a lot
more work than to explain. :-))

Roger (K8RI)


I have a chamber I use now for another purpose, but it is only 6" in
diameter. The top for it is just 3/16" tempered glass.

Hope the number might help your estimates.

Max

On Thu, 01 Nov 2007 17:38:59 -0500, cavelamb himself
wrote:

Maxwell wrote:

"Mark Hickey" wrote in message
...

Fred the Red Shirt wrote:


How big is his vacuum chamber?

What sort of vacuum pump(s) does he use?

One large enough for a 17.5" mirror is rather non-trivial. Assuming
a 20" diameter cylindrical chamber, the top and bottom would need
to support over 3000 pounds each, if the work is done at sea level.

Is it really that hard to build a vacuum chamber? Seems to me that
the most pressure it'll ever experience is about 15psi (1 bar), while
it's trivial to build/buy pressure containers that can handle 10-100x
that much (positive) pressure. Certainly if building a 1 bar vessel
20" in diameter is daunting, building a submarine (or worse, a
deep-sea bathyscaphe, which have reached depths of almost 36,000 feet
below sea level, resisting a pressure of about 1,100 bar) would be
unthinkable.

Or am I missing something?



Maybe, maybe not. Round pressure vessels keep their shell walls in tension,
hence the more pressure the better they hold their shape. Vacuum vessels are
just the opposite, and quite often much easier to collapse than one might
naturally assume.

I can say I once built a round vacuum chamber out of rolled 1/4" aluminum.
It was approximately 18" long and 18" in diameter. The bottom was 3/8"
aluminum, and the top was 1" clear plastic. The chamber was successful with
up to an near perfect vacuum, and used many times without failure. At
maximum vacuum, the bottom would dish approximately 1/8 to 3/16" inch, an
the plastic top would dish about 1/2".

I have a chamber I use now for another purpose, but it is only 6" in
diameter. The top for it is just 3/16" tempered glass.

Hope the number might help your estimates.

Max




You know, an ABSOLUTE vacuum is only 15 psi...
But an 18" diameter hemisphere has how many square inches? ( forgot
the formula for the surface area of a sphere) The sides of the
chamber being 18" tall and 18" in diameter would have over 2000 sq
inches X 15 or by my calculator 30,536# Add to that the area of the
half sphere that comprises the top dome and it's quite a bit of
pressure. There's a reason these things are covered with a heavy SS
mesh.

In all my years of working around that kind of equipment I've only
seen one chamber let go, but that was memorable. :-))

Roger (K8RI)

On Nov 2, 5:08 am, "Roger (K8RI)" wrote:
On Thu, 01 Nov 2007 17:38:59 -0500, cavelamb himself



wrote:
Maxwell wrote:

"Mark Hickey" wrote in message
. ..

Fred the Red Shirt wrote:

How big is his vacuum chamber?

What sort of vacuum pump(s) does he use?

One large enough for a 17.5" mirror is rather non-trivial. Assuming
a 20" diameter cylindrical chamber, the top and bottom would need
to support over 3000 pounds each, if the work is done at sea level.

Is it really that hard to build a vacuum chamber? Seems to me that
the most pressure it'll ever experience is about 15psi (1 bar), while
it's trivial to build/buy pressure containers that can handle 10-100x
that much (positive) pressure. Certainly if building a 1 bar vessel
20" in diameter is daunting, building a submarine (or worse, a
deep-sea bathyscaphe, which have reached depths of almost 36,000 feet
below sea level, resisting a pressure of about 1,100 bar) would be
unthinkable.

Or am I missing something?

Maybe, maybe not. Round pressure vessels keep their shell walls in tension,
hence the more pressure the better they hold their shape. Vacuum vessels are
just the opposite, and quite often much easier to collapse than one might
naturally assume.

I can say I once built a round vacuum chamber out of rolled 1/4" aluminum.
It was approximately 18" long and 18" in diameter. The bottom was 3/8"
aluminum, and the top was 1" clear plastic. The chamber was successful with
up to an near perfect vacuum, and used many times without failure. At
maximum vacuum, the bottom would dish approximately 1/8 to 3/16" inch, an
the plastic top would dish about 1/2".

I have a chamber I use now for another purpose, but it is only 6" in
diameter. The top for it is just 3/16" tempered glass.

Hope the number might help your estimates.

Max

You know, an ABSOLUTE vacuum is only 15 psi...

But an 18" diameter hemisphere has how many square inches? ( forgot
the formula for the surface area of a sphere)

To estimate the compressive force (imposed by pressure on the
top) on the cylindrical sides of the bell-jar use the cross sectional
area, not the surface area. The lateral component is carried by
the top, only the vertical component is carried to the sides.

The sides of the
chamber being 18" tall and 18" in diameter would have over 2000 sq
inches X 15 or by my calculator 30,536# Add to that the area of the
half sphere that comprises the top dome and it's quite a bit of
pressure. There's a reason these things are covered with a heavy SS
mesh.

In all my years of working around that kind of equipment I've only
seen one chamber let go, but that was memorable. :-))


--

FF

On Nov 2, 4:59 am, "Roger (K8RI)" wrote:
On Wed, 31 Oct 2007 20:32:08 -0700, Fred the Red Shirt
...
He has a quartz bell jar that is (again IIRC) about 18"in diameter and
close to 18" tall or a bit more. Ultimate vacuum is probably on the
order of 1.2 or 1.3 X 10^-6 or -7 Torr although it doesn't need to go
that high.



That sounds like several thousand dollars of equipment.

I think I'll look for someone else to do it for me. They usually
overcoat them too, though if I were doing my own, more frequent
recoating would be plausible.

--

FF