evaluating sdbd. crown & bridge downbearings in a new piano

[Delwin D Fandrich]

It is fairly clear that Brian knows what he is talking about here, but I
would like to clarify one or two points.

    1)  In the first illustration below, he states "...it puts the
soundboard and the ribs at odds with each other, and provides way too much
internal tension."  The word should be compression, not tension.  Somewhere
along the line of piano evolution the idea came along that tension was
somehow desirable in the soundboard panel.  Several writers have (and still
are) mis-using the word.  Compression-crowned soundboard assemblies place
the panel under compression, not tension.  On the top, the bottom, in the
middle --
wherever you care to measure them, they are under compression.  This is true
until either internal cell compression has exceeded the fiber stress
proportional limit and the cells have been permanently damaged or until
long-term creep has permanently altered the shape of the wood cells (without
necessarily damaging them.  The top of the rib will be under tension, but
not the soundboard panel.

    2)  Illustration #2 is also pure compression crowning.  Regardless of
how it is obtained, if the soundboard assembly depends on compressed wood to
either form or maintain crown, it is compression-crowning.

    3)  Even in this case, the soundboard panel will be under some
compression most of the time.  As the soundboard panel is being glued to the
ribs, the top surface of the panel might be under some slight tension, but
as it is loaded by the downforce of the strings, the entire panel will be
under some slight compression.  This will generally be true except when the
soundboard panel is exposed to very dry climactic conditions.  During more
humid periods, the panel will be under slight compression.

(And Brian, why build a "press?"  In our shop we still use the same
individual air clamps that I built some 15+ years ago by choice.  They are
far more versatile than any fixed press would be and they do an excellent
job of applying even and uniform pressure to the rib while gluing.  What
more could you ask?)

Del


----- Original Message -----
From: Brian Trout <btrout@desupernet.net>
To: <pianotech@ptg.org>
Sent: Tuesday, September 21, 1999 6:23 AM
Subject: Re: evaluating sdbd. crown & bridge downbearings in a new piano


> <Roger writes:...which of the above processes matches our method?
>
> It's a little hard to tell with just the information you've given.
>
> If you will allow me, I'd like to share some thoughts.
>
> Soundboard crown can be achieved in at least 3 different ways.
>
> 1)  Gluing a flat rib onto a very dry soundboard, clamping them flat while
> the glue dries,and allowing the added moisture content that the board will
> soak up to swell the soundboard, and provide the crown in the board.   In
my
> opinion, it puts the soundboard and the ribs at odds with each other, and
> provides way too much internal tension.  But that's compression crowning,
> design feature or not...
>
> 2) Gluing a flat rib onto a dry soundboard, and clamping them with a crown
> while the glue dries, allowing both compression crowning and perhaps a
> hybrid variety of rib crowning.  I have a problem thinking that those glue
> joints are going to hold up over the next 50 - 70 years.?  But again,
> there's a lot of internal forces working against each other within that
> soundboard / rib set unit.
>
> 3) Gluing a machine crowned rib onto a fairly dry soundboard and clamping
> them with the desired crown while the glue dries.  In this setup, there
> isn't nearly the tension between the ribs and the soundboard.  There is
> crown.  You've built it in.  The major tension on the board will arrive
when
> there is a load (downbearing) placed upon it.
>
> There are a number of other factors which will affect the overall crown of
> the board in the piano such as whether the board was dried down again
before
> it was glued to the rim, and whether the downbearing is very light or very
> heavy... the list could go on...
>
> There may be other variations I haven't thought of.  But I've been very
> happy with #3.  That's the only way I've ever done them.  From time to
time,
> I tweak a bit here and there, but that's the basic idea.
>
> Actually, I need to build a press.  I've been using individual rib clamps
> which resemble the clamps in the presses I've seen, only smaller.  Another
> project for my to do list...(there's another list that has no end...)
>
> Good luck,
>
> Brian Trout
> Quarryville, PA
> btrout@desupernet.net
>
>
> ----- Original Message -----
> From: Roger C Hayden <rchayden1@juno.com>
> To: <pianotech@ptg.org>
> Sent: Monday, September 20, 1999 10:11 PM
> Subject: Re: evaluating sdbd. crown & bridge downbearings in a new piano
>
>
> > Please explain:
> >
> > "compression crowned, rather than rib crowned, soundboards. "
> >
> > We made a pneumatic soundboard press, it works well but which of the
> > above processes matches our method?
> >
> > Roger Hayden, RPT
> > Clarks Summit, PA
> >
> >
>
>