evaluating sdbd. crown & bridge downbearings in a new piano

[Frank Weston]

OK engineers:

The soundboard assembly must withstand a substantial amount of downbearing
force, no matter how it is constructed.  A so called "compression" crowned
soundboard in an unloaded state will have the fibers in the top half of the
board subjected to tension and those in the bottom subjected to compression
(it's the rule, you got to have both).  As downbearing load is applied, the
tension in the top half of the board will be decreased, as will the
compression in the bottom half.  This decrease in internal stress in the
board assembly is what provides the resistance to downbearing.

A "rib" crowned soundboard assembly will have little to no internal stress
in the unloaded state.  It is essentially built to shape.  As a "rib"
crowned soundboard assembly is subjected to downbearing forces, the top half
of the board will be subject to a compression load, and the bottom half will
be subjected to a tension load.  The absolute magnitude of these loads (all
other things being equal) will be exactly the same as those removed from a
"compression" crowned soundboard when it is subjected to the same
downbearing.

In theory, if a "compression" crowned soundboard is loaded to a perfectly
flat shape, it will have no internal stress.  On the other hand, a "rib"
crowned soundboard under downbearing will have all of the internal stress
the "compression" crowned board did have before it was loaded.  So, the
"rib" crowned soundboard, in its working configuration will be under much
more stress than the "compression" crowned soundboard.  Maybe those
traditional builders who use compression crowning really know what they're
doing after all.

What causes "compression" crowned soundboards to fail is poor selection of
materials, improper control of humidity during assembly, poor assembly
techniques and, finally, cyclic loads over time.  There is nothing wrong
with the concept that can be improved upon by "rib" or even "hybrid"
crowning.  I know I'm pissing into the wind here, but there are some pretty
successful manufacturers who agree with me.


Frank Weston


-----Original Message-----
From: Brian Trout <btrout@desupernet.net>
To: pianotech@ptg.org <pianotech@ptg.org>
Date: Tuesday, September 21, 1999 9:31 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
>>
>>
>