REFLECTOR:weak gas struts / beating a dead horse dept.

[Chuck Jensen]

Wow, I knew I should have paid attention in H.S. physics.  Your comment is
accurate that oil can not be compressed (or water for that matter), however,
oil nor water can be "stretched" or expanded until enough force is exerted
to exceed the vapor pressure of that material.  In other words, water and
oil are "solid" both ways until the vacuum is great enough to "boil" the
water or oil by reducing vapor pressure.  Remember how water boils at a
lower temp in Denver than it does in Gainesville, FL (where everyone is
boiling over the thumping Iowa gave them...and good riddance).

I've seen a number of calcs thrown out about how many pounds of force can be
applied given areas of a piston, et all.  However, no one has concluded that
this vacuum force is anywhere near sufficient to vaporize hydraulic fluid.  

Finally, going back to the syringe example, yes, if you pull on it, it will
come out, but that is a function of the lousy seal in the syringe, low vapor
pressure of water, likelihood that there was a small amount of air still in
the syringe and/or liquid in syringe contained some dissolved gas which came
out of solution with a small tug.  

It is still correct that if you take a hydraulically locked system (no air,
no water or gas contamination, no seal leakage), that the plunger will NOT
move until you pull with a very large force; one great enough to vaporize
the hydraulic fluid.  Given that the vapor pressure of hydraulic fluid is
very close to a perfect vacuum, it will take very large, even massive,
mechanical force to form a bubble.

I'm pretty sure this is all true, but then, I didn't think they'd find
Saddam in a spider hole either.  

Chuck Jensen


-----Original Message-----
From: reflector-admin@tvbf.org [mailto:reflector-admin@tvbf.org]On
Behalf Of Dave Black
Sent: Friday, January 02, 2004 5:46 AM
To: reflector@tvbf.org
Subject: REFLECTOR:weak gas struts / beating a dead horse dept.


> Al,

> Well; it may be a hard mental concept, but, of course, it will
> move.  

You are absolutely correct. I've done it many times with a hypodermic
syringe. 

But I can understand why some might believe movement of the piston/plunger
impossible. Until you pull with force equal to atmospheric pressure times
area
of the piston, no piston movement whatsoever occurs. This can give the
impression that it's not going to move no matter hard you pull. However,
once
you overcome the force of atmospheric pressure (and create a vacuum),
movement
will occur with little additional effort. 

That's because vacuum is simply zero air pressure. It's impossible to suck a
vacuum below zero pressure. If you have a piston with an area of 1 square
inch, with zero pressure on one side of the piston and 15psi (sea level
pressure) on the other, you'll need 15 pounds of force to move the piston
off
its stop.

It's exactly the same as if you were lifting a 15 pound dumbbell off the
floor. 5 pounds of lifting force wouldn't budge it. Nor would 14. It's still
on the floor. But once you lift with 15 pounds of force it begins to move.
Apply 16 pounds, and it'll move upward fairly quickly.

Don't be confused by the oil. Unless the oil in this hypothetical hydraulic
system has some glue properties, the most it can 'suck' is 15psi. Beyond
that,
the piston's going to move. 

Note that all of this pertains to closing off the intake port on the suction
side of the piston. The pressure side is a whole different matter. You
cannot
compress a fluid, so closing off the exit port would absolutely stop the
piston. Until something breaks.

Dave Black
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