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MMD > Archives > July 2001 > 2001.07.26 > 07Prev  Next


Pneumatic Pouch Forces
By Craig Brougher

I was wondering from Johan's work regarding the computation of pouch
forces if he has taken into consideration the practical forces actually
experienced from the lifter disk which is purposely placed on top of
the pouch to be raised at its edges by the ballooning of the pouch
below it?

I have measured these actual force differences and they are measurably
different than that of a straight pouch and its area of actuation using
a minimal lifter spot.  So force times area isn't applicable in
practical pneumatics, unless the pouch moves like a loudspeaker (and
even then, only a bare approximation).  This is because a stiff pouch
-- at least, stiff enough not to balloon -- will also resist movement,
and the force will be largely lost.

When a pouch inflates, you might say that its combined force (radial
vectors) changes with the chord angle to the tangent line of the torus
cross-section.  So when that tangent line makes an angle to the
vertical greater than 90 degrees, no force is exerted, except to
balloon the pouch.  That means, the pouch first has to assume its
shape.  It is not "lifting immediately."

As Johan said, there is much to it, but when you place a disk at the
points the pouch would normally balloon at first, you prevent it, and
the lifting then has to take place immediately on the lifter disk.  The
larger the disk, the quicker this happens, because more force can be
employed.  However, the slower it returns, because more force has to
be used to flatten it down, out of the way.

Craig Brougher

 [ In his paper Johan states that the discussion is limited to only
 [ the forces developed by a pouch of "a flexible material that does
 [ not resist movements appreciably within the nominal displacement
 [ range."  The ballooning of the pouch is modeled as a circular
 [ section.  I think the disc is considered stiff and weightless.
 [ See the paper at http://mmd.foxtail.com/Tech/pouchforce.html
 [ Empirical (measured) data would be welcome for comparison.
 [ -- Robbie


(Message sent Thu 26 Jul 2001, 14:44:27 GMT, from time zone GMT-0700.)

Key Words in Subject:  Forces, Pneumatic, Pouch

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