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

Dave Black reflector@tvbf.org
Fri, 02 Jan 2004 05:46:15 -0500


> 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