Music Trade Review -- © mbsi.org, arcade-museum.com -- digitized with support from namm.org
JULY 26, 1919
THE MUSIC TRADE
REVIEW
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A Better Understanding of th^Operation of the Bellows System and of the
Function of the Spring Exhausf^r Will Serve to Clear Up Many Fallacies in
the Minds of Those Who Do Not Fully Comprehend the Player Action
It is unfortunate that the generality of those*
who make their living in connection with the
• player business should be so appallingly ignorant
of the fundamental principles upon which the ac-
tion of the mechanism depends. Not only do the
prevalent professional ignorance and confusion
of thought tend toward a general and parallel
confusion in the public mind with reference to
everything that has to do with the player-piano,
but they afreet also the practical ideas of those
who have to deal with the instrument as its re-
pairers and so-called "experts." Indeed, the ad-
vertising, the literature and the conversation of
almost everybody who is interested, as a layman
or otherwise, in the development of the player,
in its promotion or in its use, are vitiated by the
perpetuation of fallacies which it should be the
first object of the pneumatic engineer to remove.
One of these fallacies has to do with the mat-
ter of spring expansion. As everybody knows,
or ought to know, the various bellows of the
player concerned with the production or regula-
tion of power, as opposed to those which con-
sume or use up the power, such as the speaking
pneumatics, all require for their functioning the
use of properly adjusted springs, either for pur-
poses of compression or of expansion. It has
for years been customary to speak of the light-
ness or stiffness of these springs, relatively to
each other and to those of individual players
which happen to be under discussion, as if in
some mysterious way the pull exerted by them
promotes or hinders ease of action, power, re-
serve, or responsiveness. In consequence much
falsehood is innocently uttered by salesmen and
much error committed by repairers, not to men-
tion such bewilderment caused to users of the
player-piano who would like to adapt their in-
struments to what they find to be their indi-
vidual requirements. It is the purpose of the
present article to point out the truth of these
matters and thus assist in puncturing some of
the grosser fallacies which cluster around the
subject, all in the hope that the light thus
thrown will illuminate some dark realms.
The Operation of the Bellows System
To begin at the beginning, let us briefly state
the purpose of the bellows system and its opera-
tion. The two pumper bellows, or exhausters, as
they should better be called, are worked alter-
nately open and shut by the action of the per-
former's feet moving on suitable treadles. Each
of these exhausters is simply a leaf bellows of
relatively large size, having a flap valve on the
inside wall nearest to the air trucks of the action
and an outside Bap on the moving or outer wall
opening to the atmosphere. Now, the atmos-
phere is a complex of gaseous matter, and it is
a property of all gases to expand indefinitely.
That is to say, if a bellows when closed con-
tains (as, of course, it does) a minute quantity
of air in the shut-up spaces between its leaves,
then when this bellows is opened the contained
air, if no more can enter from the outside, will
expand to fill the enlarged space. The atmos-
phere has weight, and this weight corresponds
to a pressure at sea level of 14^4 pounds per
square inch of surface upon which it presses.
Hence the thinning or dilatation of air de-
scribed as happening when the bellows are
opened is acfcQ&ipanied by a fall in pressure; or,
in other words, as the contained air is
stretched or thinned, any given cubic inch of it,
to take a concrete example, will be lighter than
it was when in a normal condition. But since
the opening of the bellows brings about a
marked dilatation of the air contained in it, and
since there is a flap both outside and inside, it
follows (1) that the outside air pressing against
the outside flap prevents itself from entering,
while (2) the air contained in the chambers and
galleries of the player action, being by hypothe-
sis of normal pressure before the bellows work
is begun, must press against and open up the
inner flap and rush into the bellows space until
the pressure therein is equal to the pressure
throughout the rest of the player. But since
this involves an expansion or dilatation in the
volume proportion of the bellows to the rest of
the player action, it follows that after the ex-
pansion and the equalization of pressure the
pressure throughout the player will be consider-
ably lower than normal.
All this happens while the bellows is pushed
open. When, now, the foot pressure on the
treadle is released a spring behind the moving
wall thrusts the same back until the bellows is
again closed. This operation of closing squeezes
the air which has expanded in the open bellows
until its pressure rises above that of the atmos-
phere, when it forces its way out against that
pressure through the outer flap. Thus at each
complete stroke back and forth of the foot on
the treadles a definite proportion of the air con-
tained in the player is expelled into the atmos-
phere.
The Exhauster Spring and Its Work
Now what part does the spring of the ex-
hauster play in this operation? It is plain that
this spring may be light or heavy, relatively
speaking, but it must be heavy enough to close
the bellows against the air pressure within, hav-
ing the assistance of the higher normal pressure
without. It is likewise plain that this function
of closing the bellows is the only function of the
spring. Now, the heavier the spring may be
the more quickly will it close the bellows, and
therefore the more rapidly can the bellows be
operated, since the quicker closing means that
the foot may open the bellows more often in
any given time. But it is also plain that the
heavier the spring the more work will be re-
quired, in the sense of physical effort, to open
the bellows against its resistance. So that it is
plain that the heavier spring simply means that
the foot, by working harder, is enabled to pro-
duce a given vacuum in less time or to produce
a higher vacuum in a given time.
Evidently, then, since the converse is true of
the lighter spring, one can only say that the
weight of the spring of itself has no effect, and
that indirectly it has the effect of permitting
slower, lighter work, or faster, heavier work, as
the case may be. Therefore it may be said in
general terms that the heavier spring is better
for those who can put the effort behind it, and
vice versa.
The function of the spring in the equalizers,
or so-called "reservoirs" (the term is incorrect),
is exactly the reverse. In this latter case the
equalizer is held open by the spring, and its
function is to impose resistance to the closing
of the equalizer under the atmospheric push
from the outside, which is realized when the in-
terior air is dilated. As soon as a "partial
vacuum" is created in the equalizer by means
of the exhausters, as described above, the atmos-
pheric push from the outside will close the
equalizer, provided the spring resistance is not
too great. Thus the heavier the spring the
greater the resistance to the work of the equal-
izer and the more slowly will the equalizer
close. But the equalizer does no work till it
has closed, at least partly, and is ready to open
again. When it begins to open it becomes an
auxiliary exhauster and steadies the work of the
regular exhausters. Now it is easy to see that
the more resistance imposed by the spring the
less chance there is for the equalizer to close
and remain closed. If the equalizer under hard
pumping remains closed it cannot do any work
as an auxiliary exhauster, and so its extra ad-
ditional force is lost and the possible vacuum
level remains lower than it otherwise would.
Hence the heavier the spring pressure the higher
will be the vacuum level and the louder one
can play. Yet, at the same time, higher spring
pressure means more physical effort required to
close the equalizer even partly, and since it
must be first partly closed in order to begin to
open, and thus to do work, it follows that
heavier spring pressure gives more power only
by giving opportunity to harder physical effort.
The Motor and Expression Governors
Similar reasoning applies to the motor and ex-
pression governors. Increase the spring expan-
sion of the motor governor and the motor runs
faster on any given position of the tempo lever.
Why? Simply because for any given force of
pumping and consequent level of vacuum the
governor closes less in proportion to the
strength of the spring. But this simply means
that, as the lesser closing implies a greater size
of passageway through which the displaced air
flows, more air must be displaced in a given time
and therefore the drag on the bellows is greater.
The same applies exactly to the expression
governor. In consequence we are compelled to
the conclusion that the player obeys the laws
of nature just as any other machine does, that
something cannot be had from nothing and that
energy is never created, only transformed.
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