High tension? Low?......Re: More on soundboard crown

[RicB]

Hi Alan.... I save quite a few posts I find interesting... this one
touches somewhat on your questions... taken from a discussion as to
whether or not the rim/frame do anything at all to support crown.
Basically the buttressed arch discussion... or nearly so.

Interesting words from Phil Ford and Ron Overs below.

Cheers
RicB


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.

-- 


*OVERS PIANOS - SYDNEY*
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