More on soundboard crown

[Overs Pianos]

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At 11:35 PM -0700 15/8/03, Phillip Ford wrote:

>. . . Perhaps 10% of the load could be taken by the arch and 90% by 
>bending.  If this were the case then the compressive stresses in the 
>board might be well below the allowables. And the rim (or tension 
>resonator, or whatever) could be said to be helping to support the 
>crown.
>
>I don't know what is actually happening.  But I would like to see 
>something other than argument or calculation before accepting the 
>dictum that the rim and frame do nothing to support crown.
>
>A proposed experiment:  Take a crowned section of panel with a rib 
>attached.  Glue one end to a rigid support.  Glue the other end to a 
>block which is guided by bushings or rollers, so that the block 
>support will resist vertical load and rotation of the edge of the 
>board (as in a piano) but will not offer any restraint for outward 
>movement of the edge (in other words will not permit an arch 
>effect). Put on load of your choice at the midpoint and measure the 
>deflection.
>
>Next take the same setup but rigidly fix the block to a support, so 
>that it can't move (in other words it will permit a thrust load or 
>allow an arch effect if there is one).  Put the same load on the 
>same spot and measure the deflection.

Hi Phil and all,

I've been developing a new sound board for our next 225 piano. 
Firstly, I built an 880 mm long CC rib to use as a control (all the 
prototype ribs are machine crowned). The first test I did was to 
investigate the assistance or otherwise of the crown, with regard to 
its ability to resist down bearing. The figures attained are included 
in the table below.



In this particular case I measured 5% less deflection 400 mm out from 
the rib end, when the test rib was clamped to the RHS steel section 
which supported the rib ends. So your guestimate of 90/10 wasn't too 
far off the mark Phil.

I also proved that sound board deflection under an increasing load is 
not linear. But the real surprise was that the rate of deflection 
increased as a greater load was applied. At least it does when 
dealing with down bearing loads up to 24 Kg on a single rib (the load 
on our rib no. 11 if the down bearing angle is at 1% when the piano 
is at pitch).

Ron O.
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OVERS PIANOS - SYDNEY
      Grand Piano Manufacturers
      _______________________

Web http://overspianos.com.au
mailto:ron@overspianos.com.au
      _______________________
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