rescaling tension

[Delwin D Fandrich]

----- Original Message -----
From: Newton Hunt <nhunt@jagat.com>
To: <pianotech@ptg.org>
Sent: June 29, 2000 7:06 AM
Subject: Re: rescaling tension


> Tension is but one of the important factors in trying to get
> a nice sounding scale.  You can play with the tension a lot
> by reducing the wrap size and changing the unwrapped ends
> around.  Balance is the key factor.  1500 pounds is only
> 0.033% of 45,000 pounds so I think you are well in the safe
> range but think about reducing it by 500 pounds or so to be
> in the save range.
>
> Newton
-------------------------------------------------------

Did that apple fall a bit hard on your head this morning, Newton?  It will
work better after a couple of cups of coffee...

That is 3.3%, I think.  Even so, it is true that this is not a significant
increase in overall tension.  However, it really depends on where the
increase occurs.  During the rescaling process it is not likely that 3.3% is
added to each individual string of the scale.  It is more likely that
somewhat more than this was added to the treble section.  Some would be
taken off of the mid- to upper tenor.  Some would be added to the lower
tenor and what goes on in the bass is anybody's guess.

I pay much more attention to what is added to the low tenor and upper bass
section than to the overall scale tension.  Usually the #1 strut and the
struts between the tenor/treble, the treble/treble and the top treble are
fairly lightly loaded relative to their size and shape.  But, as I said
earlier, the X between the bass and tenor sections is (structurally) the
weakest part of any plate design.  I get real cautious here.

I have encountered more than a few pianos in which the 'mathematically
ideal' scale -- i.e., the one that has the so-called best fit of all those
nice curves and things -- has actually reduced the overall string tension
load while increasing the localized load considerably across the break.  One
of the worst I can recall would have added about 30% to 35% to the stress at
the X while keeping the overall tension load approximately the same.  (The
bi-chords in the bass and the low tenor would have increased quite a bit
while the low bass and the mid- to upper tenor would have decreased a lot.)
That is a significant increase of stress on the X.  Easily enough to break
some of them.

With my scaling spreadsheet (a simple little thing running in Excel) I have
it set up so that I can easily sum the tensions through the upper half of
the bass section and the lower 10 unisons of the tenor section so that I can
compare the new scale with the original at this point.  I then compare any
increase in scale tensions through this region with my evaluation of the
plate.  If there is any question at all in my mind about the strength of the
X I will either back off on the tensions or add a stress coupler between the
plate and the belly rail.  Assuming the piano is worth the effort.

Regards,

Del