More on soundboard crown

[Phillip Ford]

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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. ...
>>
>>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...
>
>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...
>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.


Hi Ron,

Thanks for sharing this info.  Very interesting.  An increasing rate 
of deflection as the load is increased is what I would expect if 
there was arch behavior going on.  As the arch is deflected down, the 
arch height is decreasing, which causes an increase in the thrust 
load at the supports.  Are those deflections in mm by the way?  Care 
to try my experiment where you put a load on the guided edge to see 
how much force is required to deflect the 400 mm point up by 0.13 mm 
(the difference in deflection between the guided and fixed case)? 
This would give an idea of the compressive stresses caused by this 
effect and whether they would cause compression set over time.

Regards,

Phil Ford
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