Crown without soundboards

[Ron Nossaman]

Hi Ed,

At 12:49 PM 12/29/97 EST, you wrote:
>Greetings all, 
>     I read with interest the postings concerning crown and the rim influence
>on it.   I have often wondered if a bridge attached to the ribs, with no
>soundboard would be able to withstand the down pressure.  I am skeptical that
>the soundboard itself, what with soft thin wood, actually provides much
>support.  

A board, dried down severely, and glued to flat ribs, will expand enough
with rehydration to form a crown, by bowing the ribs, that, in the piano,
will support both the full downbearing pressure of the strings, and the
considerable force of the ribs trying to straighten back out. It's terribly
hard on the board, but it works for a while, and many soundboards have been
crowned this way. The board can, indeed, do all the work.

A better way is to machine the top of the ribs to a crown, dry the panel a
lot less severely than the first method, and glue it to the ribs. The panel
expands a bit with rehydration and bows the rib, slightly increasing the
crown above what was originally machined into the rib. In the piano, the
downbearing load is shared between the compression of the board, and the
stiffness of the ribs. This is more ideal because no single part of the
assembly is taking the bulk of the abuse.  

You could certainly machine a smaller radius crown in the ribs, not dry the
board at all, and bend the board to conform to the shape of the rib. Under
load, the ribs would be supporting almost all of the load, but the top
surface of the board would be under tension and, it would seem to me, be
more likely to crack when dried out in the winter. But it should work.

In all these cases, the board is crowned outside the piano. The balance of
forces that maintains the crown is between the panel and the ribs. The rim
is just a solid termination for the edges of the board for sound production.    



>   That a board is still crowned with glue joint failure around the rim
>doesn't tell me much.  Would not the joints of the ribs to rim actually have a
>lot more bearing on  crown maintenance? 


Not unless the ribs change length.



>   From a structural standpoint,  the principle  of arches seem to describe
>the transfer  and containment of bearing pressure.  When you vector the
>downward force from an arch, you see that there is a transfer of pressure from
>straight down to a more lateral direction, where the pressure meets the case,
>( and any centripedal apparatus that may be in the area.......(:)}} 


Ah but the pressure you are talking about doesn't meet the case. It is
anchored to the rib. The rib is the foundation for the crown. When you
install a soundboard in a piano, you are dropping in a pre-crowned, ribbed
assembly and gluing it down to the rim. There is no side pressure on the rim
to speak of even after the piano is strung. The fit between the soundboard
panel and the case side is cosmetic. It's not load bearing.


>   So,  has anyone ever considered that the ribs alone might support a bridge,
>and the soundboard itself  does very little? 

Depends on the crowning method.


Or to ask another way,  how long
>would a board remain crowned if there were no ribs?  
>Regards, 
>Ed Foote 
>

Until you tried to string it.


I hope this helps.   
 Ron Nossaman