[ Howard Kohlbrenner wrote in 250804 MMDigest:

> I am rebuilding an Autopiano double-valve three-tier player action
> that was previously rebuilt. I'm finding pneumatic spans between
> 1-1/8" and 1-1/4". The question is, what is the correct span?

The answer is that the span is related to the width of the wood.
As the pneumatic collapses, the folds of the pneumatic cloth fold
inwards and at full closure of the pneumatic, the two creases should
not touch each other. So, if the span is different, the wood width
would also need to be different.

There is a paper in the MMD Archives on the subject of the force
development of pneumatic motors, and this shows that the initial
closing force is quite high due to the cloth being straight at the
fully open position.

The span of the pneumatic is related to the dip of the key as the
piano is played, and if the key dip is the same across the full width
of the keyboard, then the pneumatic span should be the same. As well,
the span should duplicate the hand played movement with a bit of
extra capability.

Good luck with your repair, and please do not hurry the job.

Paul Rumpf
Melbourne, Australia
rumpf.paul@yahoo.com.geentroep [delete ".geentroep" to reply]

[ Paul's technical paper, "Mechanical Forces Developed
[ in Pianola Pneumatic Motors", can be downloaded at
[ http://www.mmdigest.com/Gallery/Tech/rumpf_1.pdf 
[ -- Robbie