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MMD > Archives > July 2001 > 2001.07.28 > 07Prev  Next


Pneumatic Pouch Forces - Empirical Data
By Craig Brougher

In a player (a Duo-Art) with a 1" working pouch diameter and a 11/16"
poppet, without any lifter diameter considered, and only Johan's
equation used, the lifting ratio between the forces against that valve
(if it were sitting on a flat, round, inside valve face) versus the
force of the pouch upward against it would be 2.3 to 1, meaning its
ratio of areas.  That will lift the valve regardless.

This translates, at 5" of vacuum, to a lifting force of .14 lbs. against
a resistance of .06 lbs. (63.5g. against 27.24g -- idealistically only.
The poppet valve itself weighs only 4.3 g. so that is inconsequential).

However, by adding a pouch lifter disk 5/8" diam against which the pouch
is able to exert force, it roughly doubles the lift, depending on its
depth into the pouch well and other factors.  But for practical
purposes, _if_ the valve sealing surface were perfectly smooth but with
zero surface tension to deal against, _then_ the lifting factor would
be about 4.5-5, which is about ideal -- but with these reservations
below.  It is these reservations below which changes everything.

When a pouch inflates, you might say that its combined force (radial
vectors) changes with the perpendicular 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."

Here again, for comparison's sake, are the empirical figures taken from
an earlier article of mine in the MMD Archives.  Notice that it depends
mainly on the kind of sealing valve leather used on the poppet, as to
whether or not the ideal lifting ratio can be maintained.  Keep also in
mind that we are idealizing this so that there is no gap between lifter
and pouch, and everything is reasonably square.  Whether it is or not,
the ratio remains the same overall.

 - - -

 [ From "Duo Art Cross Valve Measurements", 971120 MMDigest
 [ http://mmd.foxtail.com/Archives/Digests/199711/1997.11.20.09.html

The following tests were made with three leathers, A, B, and C.
Leather A is .030 thick, hard-bodied calfskin with an ultra-fine nap
(I call it 'the pink stuff').  Leather B is a softer handed deerskin
with a medium (average) nap and is .060 thick.  Leather C is also
deerskin, soft, medium nap, and .070 thick (from a different skin).

The weights recorded are on a Pelouse P-10  scale which is accurate
to 5 g.  So if a reading fell between, I would add two pennies.  If it
went up to the next 5 gram increment, I assumed it was a bit over its
stated value, and if it did not move, then it was either a bit under,
or exactly on that value.  However, 5 g plus or minus is meaningless
in these measurements.  The leather, as you will soon realize, is the
determining factor, altogether.  The leather you pick means the valve
will either play well or not!  There are also other sub-factors which
we will touch on at the conclusion of this article.

The valve plates are designated (O) and (X) respectively, below.  Three
vacuum pressures were recorded, 5, 10, and 15.  Three leathers were
used, A, B, and C.  Leather A is .030 extra fine nap calf, very solid.
Leather B is soft deerskin, .060, medium nap.  Leather C is soft
deerskin, .070, fine nap.  All leathers are very nice material, and
airtight.

[ The first column, 'L', is Leather sample. ]

L  |    5" vac.        |    10" vac.          |    15" vac.
   |                   |                      |
A  | (X) 15g  (O) 15g  | (X) 35-40g (O) 40-45 |  (X) 45-50g  (O)  65-70
B  | (X) 15g  (O) 15g  | (X) 35g    (O) 45-50 |  (X) 50-60g  (O)  60-65
C  | (X) 15-20 (O) 20g | (X) 30g    (O) 40-45 |  (X) 155*g   (O)  60-65

 - - -

So when we look at empirical measurements (go to the article to get the
whole story), we see that the force required to lift different kinds of
leather off a cross valve and a round valve vary greatly within the
same valve style, just by varying the kind of leather you use.

This is the most valuable, useful evidence, and when combined with
Johan's research (who is a fine scientist), you then can see that the
_only way_ that a poppet valve the size of an old Duo-Art valve (4.3 g
weight) could be stopped from moving when it's lifter disk disappeared
and only the pouch itself could lift it, is if there was quite a gap
between the bare stem and the pouch's surface, and if the leather
chosen to seal the inside valve hole was soft, thick, and had no suede
"spring."

Craig Brougher


(Message sent Sat 28 Jul 2001, 15:14:46 GMT, from time zone GMT-0700.)

Key Words in Subject:  Data, Empirical, Forces, Pneumatic, Pouch

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