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

[Alex Balic]

-----Original Message-----
From: reflector-admin@tvbf.org [mailto:reflector-admin@tvbf.org]On
Behalf Of Richard@Riley.net
Sent: Friday, January 02, 2004 4:26 PM
To: reflector@tvbf.org
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."


>>>>>I would define massive in this case as the  of thousands of pounds
range, perhaps even the tens of thousands of pounds range..


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.

>>>>>>>  This is an interesting and thought provoking question though, but
it would be one suited for a pneumatics lab rather than a hydraulic one-
just like you can compress any pneumatic cylinder with enough force to
overcome system pressure- even if both sides of the pneumatic cylinder are
valved closed. but not so with a hydraulic cylinder- liquids and gassed have
very different properties even though they are both fluids- gasses are
compressible, liquids are not- in order for our theoretical hydraulic
cylinder to move, it must contain a gas, which can be produced over time if
the fluid is volatile, but not (for practical purposes) if it is hydraulic
oil- there is a phenomenon of cavitation in hydraulic pumping circuits, but
it almost always involves either contaminated (water containing) fluid, or a
leak in the suction side of the circuit that allows for the introduction of
air into the system. I have seen some cavitation damage to the vanes of a 20
hp high speed pump once, and it was suggested that the pump cavitated the
fluid (but that was an 8,000 psi differential pressure (massive) but even
then we traced it down to water in the system.....

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.

>>>>>>> you might "significantly" lower the boiling temperature of water
with this pressure, but you won't have any effect on the hydraulic oil This
would only occur with a high vapor pressure fluid like ether and (with a lot
more work) water, if you lower the pressure enough to lower it's boiling
temperature to ambient temperature.

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.

>>>>>>> that is correct, because the hydraulic fluid is already a liquid,
(not a gas, which is compressible)


I do enjoy the stimulating conversation that this overworked thread is
producing, feel free to continue your case Richard...   :))

Interesting also that even in Iraq, where few things are done correctly, at
least they know what belongs in a cesspool..

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