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MMD > Archives > March 2012 > 2012.03.05 > 05Prev  Next

Vacuum Reservoirs Are Different Sizes
By John A. Tuttle

Hi all,  I've tried to rationalize lots of different bellows
configurations.  Some seem to make more sense than others, and some
seem to make no sense at all.  Often the springs in the reservoirs
have different tensions, with the heavier springs on the 'stack' side
and the weaker ones on the 'air motor' side.  I feel fairly confident
that the engineers had a good reason for using different springs, but
I've never been able to figure it out.

When you consider that the reservoir bellows are connected directly to
each other via the wind truck, the fact that one bellow will collapse
faster (or more completely) than its 'partner' at a given vacuum level
doesn't seem to serve much of a purpose.  It's always been my 'feeling'
that it has something to do with being able to create 'accents' when
the music is already being played fairly loudly.  In such a case, the
weaker bellows would be (perhaps) fully collapsed already (providing
a nice steady vacuum level) while the heavier bellows would be (perhaps)
half-way collapsed, making it possible to create the accent by pumping
one pedal very quickly, or in a 'staccato' manner.

In some models of the Lauter-Humana, there is a bypass valve which
senses a rapid increase in the vacuum level and the higher level is
diverted directly to the stack instead of being 'stored' (or equalized)
in the reservoir.  The bypass valve is only on the left reservoir,
which makes one think that it would have a greater influence on the
bass end of the stack.  However, since the vacuum is supplied to both
ends of the stack (and the stack is not split), in actuality, notes in
either the bass or the treble will be accented.

As for the reservoirs being different sizes, I'd want to know if the
springs in the bellows are the same.  My gut tells me that the smaller
bellow probably has heavier springs, which, as explained above, would
help create accents at higher player volumes.

In closing, it seems unlikely that someone purposely changed the
original size of one of the bellows.  Doing so would necessitate
relocating the mounting screws and fabricating a different gasket.

John A Tuttle
Brick, New Jersey, USA

(Message sent Mon 5 Mar 2012, 15:30:45 GMT, from time zone GMT-0500.)

Key Words in Subject:  Are, Different, Reservoirs, Sizes, Vacuum

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