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MMD > Archives > May 2013 > 2013.05.24 > 04Prev  Next

"How Does It Work?"
By John A. Tuttle

[ Larry Schuette wrote in 130514 MMDigest, Building a Transparent
 [ Player Piano Action Model, "I always get into a discussion with
 [ people on just how a player piano works."

Hi All,  Having just re-read Craig Brougher's chapter on Valves
and Pouch Cutouts (Chapt. 7, The Orchestrion Builder's Manual and
Pneumatics Handbook), I'm left with the feeling that it _would_ take
someone with a degree in physics to create the "optimum" player
mechanism.  But as Craig correctly points out, even if you could
create the perfect system, it would only be perfect for a particular
application, or piano.  And, even though all upright and grand pianos
are _basically_ the same in terms of their basic construction, there
are numerous different action designs.  That being the case, it only
stands to reason that the player system would have to be 'married' to
the piano action.

That said, the question 'why do players work' became a little clearer
as I read through his ten 'Aspects of a fast, efficient valve' and the
section on 'Valves in practice'.  Two of the comments that caught my
attention were, "Valve 'size' is _not_ the diameter of the valve disk.
It is just the _clamping area or contact area_ of the valve which
determines the psi (pounds per square inch pressure) which the pouch
must overcome.", and, "It doesn't matter if the valve seat has only a
small hole, if the valve disk and overall seat area is large and flat,
the weight of the poppet from pressure after it seats itself will be a
function of this total seated area."

These comments made me realize that there is, in fact, a relationship
between the 'working area' of the pouch and the 'working area' of the
valve.  However, it didn't offer any comparative ratios which might
help explain the point at which the pouch doesn't have enough power to
overcome the pressure holding the valve closed.

Another point that Craig's chapter helped clarify was the relationship
of the bleed size to the "on" and "off" speed of the valve.  To me, the
most interesting comment was, "Basically, the smaller the bleed, the
faster a valve will actuate and the slower it will return."  And along
that same line, he also discusses the 'pouch to stem' distance and the
forces working against the return of the valve to its seat.  He writes,
"The 'return trip' includes vacuum losses and a slightly resistive,
pillowly pouch which must be at least partly collapsed by the returning
valve stem in order for the valve to reseat.  As little as the return
pressure is, the poppet weight is still many times less than the vacuum
pressure attracting the poppet to seat."

The above two points indicate a measurable and direct relationship
between the bleed size, the 'pouch to stem' distance, and the
repetitive capability of the valve.  While this is relatively common
knowledge to the educated player technician, it points to the fact that
there must be a formula for determining optimum performance within a
set of specific values.  Also, these are only two of the ten aspects
which Craig mentions at the beginning of the chapter.  Obviously, as
he also states, every change you make in the construction of the valve
will have some sort of an effect on its performance.

Lastly, for the time being, I've also started gathering information
about the comparison between air pressure and vacuum pressure.  While
it might seem that they are easy to explain, I'm finding that is not
the case.

One question I have been unable to answer is; what is the relationship
between a pound of air pressure and an inch of vacuum.  I can't seem to
locate a formula to converts [water gage] inches to pounds per square
inch.  And, while it would seem obvious that there is no correlation
between the two, if you have 14.7 psi on one side of a flexible
membrane and 13.7 psi on the other, the membrane will move in the
direction of the lower pressure.  So, how many inches of vacuum is
equal to one pound of negative air pressure?

I think the answer will help explain "why" the pouch is able to push
the valve open with relative ease even in light of the fact that
there's a bleed which is constantly trying to equalize the vacuum level
on both sides of the pouch.

John A Tuttle
Brick, New Jersey, USA

 [ This table may be helpful:
 [ --Jody 

(Message sent Fri 24 May 2013, 19:18:02 GMT, from time zone GMT-0400.)

Key Words in Subject:  Does, How, It, Work

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