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MMD > Archives > December 2007 > 2007.12.20 > 05Prev  Next

Normal Suction at Player Piano Reservoir
By John A. Tuttle

Hi All,  The question arises as to what vacuum level (in inches of
water) is normal for a foot-pumped player piano.  Since "normal" is
a subjective word, the answer is likewise subjective.

In all regular 88-note player pianos, the vacuum level at the tracker
bar is exactly the same as the vacuum level in the stack at any given
moment in time.  This is because the bleed which supplies the vacuum
to the tracker bar gets its vacuum from the inside of the pouch well.

Aeolian Corporation established a test back in the 1970's for
determining if an exhauster assembly met minimum operating standards.
They said that if the assembly held a vacuum for 12 seconds, it passed
the test.  Mind you, this is a _minimum_ requirement.

Today, depending on the type of exhauster assembly in question,
most rebuilders shoot for 30 seconds or more.  Naturally, the fewer
'built-in' leaks that the assembly has, the longer it should hold a
vacuum.  (A number of exhauster assemblies incorporate a small leak to
prevent the air motor governor from going into 'lock-up'.  Some others
have a built-in leak when the Tempo is set to "0".)

The vacuum level at the 'mid-point' (when the reservoir bellows is
half way open) is established by the springs inside the reservoir
bellows.  And, since most manufacturers used a variety of different
springs, there is no "constant" or consistent value at the 'mid-point'.

As far as a maximum vacuum level is concerned, the answer to the question
should be "How hard can you pump the pedals?"  But, most people stop
pumping when the reservoirs are completely collapsed.

Should one pump the pedals harder, what they are really checking is the
maximum attainable level before leakage reaches its maximum.  In other
words, in theory, if you can push the pedals hard enough to attain 100
inches of vacuum, then if the assembly has _no_ leaks, you can get
reach the 100 inch level.  But, realistically speaking, a good assembly
should be able to attain a level of 28-32 inches when a pedal is pushed
extremely hard.  That level of vacuum should produce a 'ffff' volume
level on any decent full-sized upright piano.

Now, back to the beginning, the exact opposite is true when it comes
to how quietly the player can play the piano without missing any notes.
That level should be about five (5) inches.  And, at that level, the
vacuum level at the tracker bar will be five (5) inches.

Lastly, and I touch on this topic with some trepidation because I don't
understand the physics, all rebuilders have noted that the main vacuum
supply tube (going to the stack) varies in size from 3/4" to 1-1/2".
I've simply come to apply what I think are suitable phrases (or terms)
for the extremes.

I call a unit with a 1-1/2" supply a "low vacuum, high volume" unit.
In other words, it will work well on a low vacuum level, but it needs
a large volume of air to do so.  Conversely, a unit that has a 3/4"
supply is a "high vacuum, low volume" unit.  In practice, this seems to
hold true, and units with large supply tubes usually have larger
exhausters and reservoirs than units with 'small' supply tubes.

However, there are exceptions, and that's why I stated that I had some
trepidation about mentioning the obvious differences.  My thought is
that it has to do more with the amount of vacuum required to empty (or
fully collapse) to striker pneumatics than any other aspect of the
mechanism...  But I'm not certain.

John A Tuttle
Brick, New Jersey, USA

 [ Since the broad pneumatics of a 3-tier action stack contain more
 [ air than do the narrower pneumatics of a 2-tier stack, it follows
 [ that the 3-tier stack is a "low vacuum, high volume" system, and
 [ so on.  -- Robbie

(Message sent Thu 20 Dec 2007, 14:52:21 GMT, from time zone GMT-0500.)

Key Words in Subject:  Normal, Piano, Player, Reservoir, Suction

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