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

J P Rourke reflector@tvbf.org
Fri, 02 Jan 2004 09:35:27 -0600


Chuck, the conclusion that this calculated force would vaporize 
hydraulic fluid is irrelevant for determining the upper bound (even if 
it didn't vaporize, a vacuum can still be formed) - "this vacuum force" 
(of area of the piston x atmsopheric pressure) is exclusive of *any* 
vapor pressure considerations; i.e., it would move even if there were no 
fluid and no vapor pressure to consider at all. So if there is no fluid 
at all, with a1.5" piston at sea level (14.7psia), it will take 25.977 
lbs-force to move it.

Once you have the upper bound established, you can conclude that with 
hydraulic fluid in the chamber, it will take somewhat less force 
(because it will vaporize at some point before an absolute vacuum is 
formed); for water it will require even less force; for ether much less.

BTW, the higher the vapor pressure, the more the substance "wants" to 
vaporize (by any method - sublimiation, evaporation or boiling). So 
ether has a higher vapor pressure. It "wants" very much to vaporize, it 
wouldn't take much pulling force for it to do so. I don't know if 
concrete really has a vapor pressure, but I can tell you it would be 
very very low!

-John




Chuck Jensen wrote:

>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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