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. ...
>>>
>>>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.

Certainly.

>   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?

Yes, the first line of data is with the rib unclamped, while the 
second line of data is with both rib ends clamped.

>   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.

There's a lot of force going outwards when the two ends are clamped. 
I know this because I was holding the rib ends with a couple of F 
clamps. I had to make them really tight or the dial gauge wouldn't 
return to zero after removing a test load. This indicated that the 
rib end was creeping along its support block when the load was 
applied, so that when the load was removed the test panel remained 
under some tension which was pulling it down (often around 0.05 to 
0.075 mm - 2 to 3 thou')

I'm am sure that as Del and Ron N. have mentioned the arch effect 
will be reduced to near zero in a very short time, after installing a 
board, due to compression set. Initial reduced board response, due to 
initial arch assistance, could go some way towards explaining why 
instruments never sound at their best when new. It seems to take 
about a year for them to really bloom. Remember that the rib I tested 
was built one day and tested the next.

I am inclined to suspect that the biggest problem in the killer area 
is that the ribs are collapsing under the extreme down bearing loads 
to which they are typically loaded. 24 Kg is one hell of a weight to 
put on a single standard rib, even if it is only 560 mm long. 
Furthermore, most manufacturers reduce the rib sectional sizes in the 
treble. Typically, the rib under the first capo section might be 
around 20 mm X 20 mm. Ribs of these dimensions have absolutely no 
hope of any service life if they are subjected to 24 Kg or more, 
which is often the case.

Best,
Ron O.
-- 

OVERS PIANOS - SYDNEY
      Grand Piano Manufacturers
      _______________________

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