Re: Glass Suction


"John Polasek" <jpolasek@xxxxxxxxxx> wrote in message
news:96vfd41kg7h4tfuji47dal3jb7opefv9pi@xxxxxxxxxx
On Mon, 22 Sep 2008 16:15:29 +0100, "Androcles"
<Headmaster@xxxxxxxxxxxxxxxx> wrote:


"John Polasek" <jpolasek@xxxxxxxxxx> wrote in message
news:4f6fd4hhddlju9ul8hrqb431t9u7khq6qh@xxxxxxxxxx
On Mon, 22 Sep 2008 07:37:25 +0100, "Androcles"
<Headmaster@xxxxxxxxxxxxxxxx> wrote:


"John Polasek" <jpolasek@xxxxxxxxxx> wrote in message
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On Mon, 22 Sep 2008 02:00:21 +0100, "Androcles"
<Headmaster@xxxxxxxxxxxxxxxx> wrote:


"John Polasek" <jpolasek@xxxxxxxxxx> wrote in message
news:h0qdd41prr8h6uqu3i918hi4tck347klsc@xxxxxxxxxx
On Sat, 20 Sep 2008 18:50:28 -0700 (PDT), eratosthenes
<rehamkcirtap@xxxxxxxxx> wrote:

I was thinking about the force that holds two pieces of clean,
smooth
glass together when they are lain in a stack.

I am however having trouble writing a quantitative expression for
the
force per area.

Any thoughts?
With glass as ideal as Jo-blocks, when you lift the top block, the
second block tries to drop away, but immediately a vacuum is formed.
Air will start to leak in but, the gap at first may be only less
than
a micron so seepage will be slow but finite and it could be several
seconds before they parted.
If you buy this you can use this arithmetic:
rho*g *h = Pressure
2600 kg/m^3, 10m/sec^2, and 14.7psi yields h = 3.9 meters. Vacuum
should hold up a 3.9 m block of glass. With a short stack it could
last quite a time.
This micron seepage business is the reason why air bearings work.
John Polasek

What's a 3.9 m block of glass, John, and why mix pounds per square
inch
with kilograms per cubic metre?


OK 100,000 Pascals will hold up a block of glass 3.9m (13 ft) deep of
any cross section whatever just as it will also hold up 33 ft of water
which is 2.6 times lighter.

John Polasek

Thank you for that clarification. Is micron seepage the reason why oiled
journals (typically found in automobile engines) work?
Don't think so. Air bearings work on sharing of pressure. The air
source is at high pressure, and the load gap at low pressure with the
essentially constant flow of air through the pores. When load is
applied to reduce the gap, the air keeps coming because of the huge
micron-caused impedance, and the gap pressure rises as its impedance
increases.
John Polasek

Did I say anything about air?

Allow me re-examine my question...
"Is micron seepage
the reason why
oiled journals
(typically found in automobile engines)
work?"

No... I did not say anything about air.
Now let me answer the question I thought you were going to ask me
but didn't. The answer is: Yes, you don't think.

Why did you suddenly reduce your requirements for a quality answer by
accepting your own answer, which is no answer at all?

I think I lit the way for the curious by showing how micron seepage
could sustain a temporary vacuum to lift a stack of glass plates, with
the offhand remark that air bearings also involve micron seepage, and
I went to some trouble to illuminate this much misunderstood
hydrostatic phenomenon, after which I opined that it had nothing to do
with journal lubrication, since there was a sharing of pressures, and
bearings are a onepiece operation.
We know that shear pressure in an oiled bearing is proportional to the
velocity gradient and it is able to drag the oil along like a turbine
pump. There is more than meets the eye and even Wiki covers it
briefly, but, your bad, you didn't look it up.
John Polasek



The answer was "yes", and totally unrelated to the answer you gave.
Unless of course you think journals run in air or vacuum, oil seeps in.
Well, I'll correct that; oil seeps in no matter what you think and an air
bearing will drag the air along like a disk drive read/write head, since it
is the disk that rotates.
http://en.wikipedia.org/wiki/Disk_read-and-write_head
Your bad, you didn't look it up.





.

Relevant Pages

  • Re: Glass Suction
    ... glass together when they are lain in a stack. ... This micron seepage business is the reason why air bearings work. ... Air bearings work on sharing of pressure. ...
    (sci.physics)
  • Re: Glass Suction
    ... glass together when they are lain in a stack. ... This micron seepage business is the reason why air bearings work. ... Air bearings work on sharing of pressure. ...
    (sci.physics)
  • Re: Glass Suction
    ... glass together when they are lain in a stack. ... This micron seepage business is the reason why air bearings work. ... Air bearings work on sharing of pressure. ...
    (sci.physics)
  • Re: Glass Suction
    ... glass together when they are lain in a stack. ... This micron seepage business is the reason why air bearings work. ... Air bearings work on sharing of pressure. ...
    (sci.physics)
  • Re: Glass Suction
    ... glass together when they are lain in a stack. ... This micron seepage business is the reason why air bearings work. ... Air bearings work on sharing of pressure. ...
    (sci.physics)