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MMD > Archives > September 1998 > 1998.09.11 > 08Prev  Next

Testing Pouch Sealants
By Craig Brougher

On page 177 in my book, The Orchestrion Builder's Manual, I recom-
mend both rubber cement and silicone grease.  Phil's suggestion,
using silicone rubber, may also be very good.  I tried egg white, too.
Too stiff for reproducers, but okay otherwise.

What you seal a pouch with isn't really all that important in a player
piano, an orchestrion, a band organ, a large organ, etc.  Whenever you
are working with moderately "large" pressures or large pouches, use
whatever you like (as long as it doesn't migrate, vaporize, get hard,
turn to powder, or whatever).  However, in a reproducing piano, your
choices are critical if you test valves at 3.5 to 4 inches of vacuum.

For those who want to know, for sure, what they're doing, I suggest
this test -- not that it's going to convince anybody otherwise, but
just for your own information:

1.  Counterbore four 1-inch diam. holes about 3/16" deep in a 1/2"
thick board.

2.  Place four pouch leathers (same thickness) over the holes and
overdip them enough that the "control" pouch without any sealant
on it will stand on its own stiffness.

3.  Glue to the centers of each pouch a little "cup", 1/2-inch diam.,
made from pre-formed aluminum foil (real light).

4.  Seal three pouches with the stuff you choose, in the way you want
to use it.

5.  Inflate all four pouches so they stay standing above their wells
with their little cups.

6.  Start laying lead bird shot with tweezers, gently, one at a time,
into the little cups until they begin to drop.  From this point on,
it's your call.

I think that everybody will see that all sealants create stiffness,
including silicone grease.  That isn't the point.  It's the degree of
tradeoff that's important.  The stiffness factor versus the tightness
factor equates to the "sensitivity" of the pouch.

Since a poppet returns with something equivalent to 1/5th (or less) the
force it took to raise it (actuation force), the stiffness of the pouch
becomes a factor as soon as the valve stem contacts the pouch again and
tries to help deflate it on the way down.  I don't know the ratios
involved here and have never seen them explained (since sealing a pouch
only helps it actuate more sensitively) but it must become the tradeoff
in valve repetition speed with any player system.  However, the proof
is always in the valve operation, and every valve type is a bit

By the way, pure silicone grease DC111 will not migrate.  That is not
true of all of them.  Grease compounds containing silicone are no good
at all, having a vapor pressure.  When the oil separates from the soap,
then it will leach into the surrounding wood.  DC111 is designed to
seal valve packing materials in harsh environments: high/low
temperatures and pressures.

Packing is considerably more porous than wood side grain, and if
this grease leached down through that packing, it would no longer be
serviceable and would allow blow-by and leakage.  Any sealant will
contaminate end grain, by the way.  I don't say it won't.  But I have,
over the years, replaced probably hundreds of pouches that weren't
flexible enough to pass valve tests and which had been sealed with
silicone grease.  No problems.

If you use rubber cement, I strongly suggest that you use a "leather"
cement designed for belts and sole cement, since it has been compounded
to be waterproof and will last much longer than general purpose cement
used for party favors and kindergarten supplies.

Craig Brougher

 [ That seems a good test, Craig; thanks for the description.  If the
 [ up/down forces could be measured and plotted I think the curves would
 [ resemble the curves which describe the force (and hysteris) of a
 [ small push-pull switch.  I'm reminded of the measurements made by
 [ Clarence Hickman on the friction of the piano action.  -- Robbie

(Message sent Fri 11 Sep 1998, 13:42:21 GMT, from time zone GMT-0500.)

Key Words in Subject:  Pouch, Sealants, Testing

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