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MMD > Archives > May 1999 > 1999.05.31 > 07Prev  Next

"Dishing" the Pouch
By John A. Tuttle

Hi All,  In my opinion, the dish of the pouch should never be less than
half the depth of the pouch well, and never so great that the pouch
actually touches the bottom of the well.

Pouch leather has a wonderful habit of conforming to it's surroundings
over a relatively short period of time.  So even if the pouch is set a
bit too deep, it will <grin> rise to the occasion.  And as shown in the
drawing of the vacuum-operated pouch tool that I sent to the MMD some
months ago, the center of the tool is exactly 1/8" lower than the rim.

I believe 1/8" is a relatively standard depth (in the center).  Of
course, the optimum depth is also effected by the size (diameter) of
the lifter disk as compared to the size of the pouch since 'squaring-
off' the center portion of the pouch will decrease the maximum possible
depth under normal operating conditions.

The basic problem I see in using a very slight dish comes years down
the road when the leather starts to shrink.  This will cause a small
amount of pressure on the valve stem which will make it more difficult
for the valve to seal at low vacuum levels.

In Larry Givens' book (pg. 42, 'Rebuilding the Player Piano'), he
recommends using a pouch setter that is 1/64" shorter than the deepest
part (center) of the pouch well.  I have found that to be slightly
excessive and prefer a clearance of 1/32" for a 1" (in diameter) pouch

In my opinion, it is better to err on the side of a bit too much dish
as opposed to too little dish.  But another consideration is the
thickness of the pouch leather.  Leather that exceeds 0.017" can
present a problem when the pouches are relatively small and the dish
relatively deep.  In situations where there is a question in my mind,
I test the pouches flexibility and suitability for use by 'puffing' air
into the well at a distance of 5-6 inches.  (What I attempt to simulate
is the action of a tracker bar signal at 5"-7" of vacuum).

If I have to 'blow' into the well to get the pouch to rise, the leather
is either too stiff or the dish too deep.  When the pouch is in the
'inflated' position, I 'puff' again at the face of the pouch.  It
should return to it's starting place without any effort.  Here again,
if you have to 'blow' at it to get it to return..  there's a problem.

The only other hint I can think of right now is (and I know this sounds
like 'old hat') duplicate the original.  Across the board, I've always
found 'duplicating the original in every aspect' to be the most
consistently successful methodology.


John A. Tuttle

P.S.  Also read Craig Brougher's article on Rebuilding Ampico
Block Valves at:

(Message sent Mon 31 May 1999, 12:27:12 GMT, from time zone GMT-0400.)

Key Words in Subject:  Dishing, Pouch

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