Crown on Compression Boards

[David Love]

What is especially alarming is when you run some numbers on how effective
rib support is in these areas (just starting to play with some toys).

A 500mm rib typical of a killer octave rib on a Steinway that has a
60'radius produces about 1.6 mm of crown.  Having just measured an original
rib on a model O in that section at 28mm wide and 19 mm high, when loaded at
1.25 degrees, as those pianos typically are, about 80 lbs of load is carried
on that rib.  The rib will deflect (according to my calculations) by 5.6 mm
yielding a remaining crown of negative 2.4 mm.  Presumably, the balance of
that support needed to hold the crown positive is carried by a compressed
panel.  Seems like not great odds, at least not for very long.  

David Love
davidlovepianos at comcast.net 

-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Ron Nossaman
Sent: Monday, May 08, 2006 2:09 PM
To: Pianotech List
Subject: Re: Crown on Compression Boards


> So the equivalent of about a 60'radius?  Will the smaller ribs of the 
> upper end of the piano, then, yield a tighter radius when forced into 
> bending mode or is it about the same proportionately?


If the treble is thinned, less. Even so, a 60' radius will get 
you about 0.3mm of crown.
Ron N