Hi All, The "span" of any pneumatic is _not_ measured from the
inside edges of the pneumatic boards. Instead, it is measured from
the outside edges. This was established many years ago to avoid any
confusion with regards to the correct span.
As a general rule, all striker pneumatics (in player pianos) have
a maximum span of 1-1/4". There are a few exceptions, but even they
are not relevant to a well-operating striker pneumatic.
With all striker pneumatics and bellows that collapse to do their
respective job, the greatest amount of force that the bellows exerts
occurs at the moment the bellows starts to collapse. From that point
on, the force it exerts decreases to zero -- when it's fully collapsed.
(Naturally, we're not speaking here about reservoir bellows that have
internal springs which keep them from collapsing. However, the same
rule applies to exhauster bellows, which generate their greatest force
when the foot pedal is initially depressed, or from the moment that
the external flap valves are fully seated.)
To my understanding, the reason for graduating the length of the
strikers in some Duo-Art mechanisms was to compensate for the weight
of the push rod. Naturally, the longer rods are heavier, and all other
things being the same, a slightly longer bellow with the same span will
exert a slightly greater amount of force. However, I'm not educated in
physics. So, I can offer no proof that my understanding is correct.
John A Tuttle
Brick, New Jersey, USA