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

[Jeffrey Clough]

----- Original Message ----- 
From: <Richard@riley.net>
To: <reflector@tvbf.org>
Sent: Friday, January 02, 2004 1:26 PM
Subject: RE: REFLECTOR:weak gas struts / beating a dead horse dept.


> At 09:40 AM 1/2/04 -0500, you wrote:
>
> >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.
>
> It all depends on your definition of "large, even massive."
>
> Imagine a hydraulic cylinder floating in space, surrounded by vacuum.  The
> piston is fully retracted. Both ports are open, so there's nothing but
> vacuum inside.  You plug both ports, and pull on the shaft, what
> happens?  It pulls out with no resistance.  The pressure comes from 2
> places - atmospheric pressure on the shaft, and atmospheric pressure on
the
> fluid in the top half of the cyl.
>
> If there's no vapor pressure to worry about - and I agree, with modern
> hydraulic fluids, the pressure is so low we can ignore it - the force is
> going to be 1 atmosphere (14.7lbs at sea level) times the area of the
> piston.  If you have a 2 inch ID cylinder, and a 1/2 inch shaft, and
you're
> at sea level, it will be 46.18 lbs (plus friction, but I'm ignoring
> that.)  At that point the fluid will cavitate, a void will form, and will
> get bigger as the shaft is pulled out.  A big force, but not that big,
> especially if there's some leverage on the shaft.
>
> Now, suppose instead of plugging the upper end of the cyl, we plug the
> lower end?  So as you try to pull the shaft out, you're trying to compress
> the hydraulic fluid?  Then, as you correctly point out, you're trying to
do
> the impossible, and the shaft won't move until you have enough force on it
> for something to burst.
>
>
> >I'm pretty sure this is all true, but then, I didn't think they'd find
> >Saddam in a spider hole either.
>
> To be totally accurate, they didn't.  It was even better poetic
>
justice...http://www.sftt.org/cgi-bin/csNews/csNews.cgi?command=viewone&id=3
36&database=DefenseWatch%20Archive%2edb
>
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