evaluating sdbd. crown & bridge downbearings in a new piano

[Roger Jolly]

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Hi Richard,
                 I find this discussion very interesting, but have little
knowlege in a real sense, but some food for thought.
  If the upper surface of the board in under tension? We then load with some
compression, down bearing. At some point we will pass' thro a state of
equilibrium.
  A question for the experts, What analysis is available, or is done in the
design phase, of these opposing forces?  Will equilibrium at say 40% RH at 70
degrees, give the optimum sustain?
  Is the equilibrium factor considered, and is it the factor in longevity?
Put another way, we have all heard wonderfull sustain from some seemingly flat
but tight boards.
The more I read and observe on the subject, the more baffled I become.
Great pianos have been built by both methods, stiring great passions. But is
the secret some where in the middle? A little less rib shaping, and a narrower
MC differential? Or a combination by register?
Now I will go back under my rock.
Roger



At 10:16 AM 22/09/99 +0200, you wrote: 
>
>   
>>
>>   
>> <<"A so called "compression" crowned soundboard in an unloaded state will 
>> have the fibers in the top half of the board subjected to tension and
those 
>> in the bottom subjected to compression">> 
>>   
>>
>>  What is an "unloaded state" in a compression crowned board?.....do you
mean
>>
>> a board with no pressure, i.e., downbearing placed on it?  A compression 
>> board by definition is "loaded" with or without downbearing...isn't it? 
>>  
>
> Intuitively, it would seem reasonable that if you take a, say 1/4inch thick,
> by 2inch wide and say 3 feet long board and bend this to some degree of arch
> and then secure the ends so that the arch is supported by the constraints
> imposed by the lenght of the secured ends, the following conditions exist: 
>
> the convex side off the board will be "stretched" (ie under tension), and
the
> concave side will be "squeezed"  (ie under compression). 
>
> When some load then is applied to the convex side of the board then it would
> be reasonable (again intuitive thinking here), to assume that the
compression
> of the board will increase on both sides. The ends are secured so they cant
> move, that would mean that even tho the concave side would "flatten out" a
> bit with the load, it would also be further "squeezed" due to lack of
> mobility at the endpoints. The convex side would  be "squeezed" in
> relationship to its present "stretched" condition. Just how much load would
> be neccesarry to bring the convex side into compression relative to its
> natural state is beyond me, but I would think it would be sometime before
the
> board was completely flattened by the load. 
>
> This would seem to describe (oversimplyfied and taken to an extreme) the
> situation in a soundboard crowned by forcing it onto a curved rib
structure. 
>
> I am wondering then just how,  this picture changes by using humidity to
> facilitate the crowning process. Something tells me that "growing" a crown,
> is bound to affect the structure of the wood differently the "forceing" the
> crown. 
>
> Also,, the difference between gluing straight ribs on a crowned board and
> gluing curved ribs on a flat board.... and what happens to each when these
> are then placed under load... Ron seemed to have the best explanation
> there... but I would like a bit more clarification. 
>
> All science aside for the moment... my gut feeling tells me that "natural"
> (excuse the term) crowning is somehow better then forced crowning, tho
> undoubtably it is more vunerable to bad climate control. Still, Rons
postings
> are very convincing.... even tho interestingly enough he ended his last
with:
>
>
> Early failure is designed into a compression crowned 
> board. The surprise is that some last as long, and perform as well as they
> do. 
>
> Richard Brekne 
> I.C.P.T.G.  N.P.T.F. 
> Bergen, Norway 
>   
>   
>   
>   



Roger Jolly
BaldwinYamaha Piano Centre
Saskatoon and Regina
Saskatchewan, Canada.
306-665-0213
Fax 652-0505 
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