Soundboard crown

[Erwinspiano@aol.com]

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In a message dated 8/16/2003 12:00:57 AM Pacific Daylight Time, 
fordpiano@earthlink.net writes:

> Subj: Re: Soundboard crown 
>  Date: 8/16/2003 12:00:57 AM Pacific Daylight Time
>  From: <A HREF="mailto:fordpiano@earthlink.net">fordpiano@earthlink.net</A>
>  Reply-to: <A HREF="mailto:pianotech@ptg.org">pianotech@ptg.org</A>
>  To: <A HREF="mailto:pianotech@ptg.org">pianotech@ptg.org</A>
>  Sent from the Internet 
> 
> 
> 
> >> >>>       Calin
>>>      As I'm sure you know the board your describing to be made with flat 
>>> ribs will not stay straight but crown once the dried panel gains moisture. 
>>> What you're describing is the old syle  pure compression type of board 
>>> making. That is unless you mean that  no panel drying is done prior to the rib 
>>> glue up. In this hypothetical case some amount of bearing is no doubt likely 
>>> to reverse the crown of the board don't you think. And the panel will 
>>> likely crack as soon as it dips below it's realitively high E.M.C at the time 
>>> of ribbing....
>>  
>> >>>   Regards---Dale
>>> 
>>      Phil Fod writes

          Dale,
>     What makes you think it will crack?  Because the board is in tension 
> from the downbearing and as it gets dryer it goes into further tension? 
>>Phil-- My thought was that any wood will shrink over time especially 
quartered wood because of it's expansion contraction properties and actually that 
could take some time. However We've all seen old stwy board with no cracks 
,compression ridging, crown or bearing for that matter that make my statement  
questionable. Or perhaps whatever little compression the wood had originally has 
equalized or was never great enough to cause compression set or damage.
   Do you think that would be enough tension to exceed the tension strength 
of 
> the board?  I wonder.  I also wonder what causes cracks in boards to occur. 
>  Do you think it's from tension exceeding the nominal allowable strength of 
> the wood?  

  >>>>Me too
       Or is it from compression damaging a section of the wood during a 
period of 
> high EMC which then weakens it so that the allowable strength (in tension 
> as well as compression) is reduced, so that the next time it sees a low EMC it 
> cracks.

>>>>>Everbody says so

>  
> I also wonder how a flat board that is pushed into reverse crown by the 
> downbearing will behave with changing humidity.  With decreasing humidity the 
> board wants to shrink.  It seems to me that this would mean that the board would 
> want to move up, back toward the flat position.  But the ribs are resisting 
> this.  So a moment would be set up between the ribs and board that would want 
> to push the board down.

>>> If it's reversed the ribs to my mind would now be acting as  a spring in 
tension and could behave the way you describe.  
        The reverse would be true with increasing humidity.  The board wants 
to 
> grow, which seems to me would mean moving downward.  But the moments 
> between ribs and board would be tending to push the board up.  So perhaps this 
> configuration is actually more stable than the conventional configuration.

... I dunno. It could be

> 
> With a conventional board having crown and with a downbearing load I think 
> the board is always in compression.  However, with this reverse crown board I 
> can imagine that there is some humidity level where the expansion of the 
> board induces a compression load in the board which just balances the tension 
> load from the downbearing so the board is in an essentially unloaded state.  As 
> Del was conjecturing in another post, the acoustical behavior of a board in 
> this state may be very different from a conventional board.

>>>I agree

> 
> Didn't you post some time ago about a piano (I think it was a Sohmer) that 
> came through your shop that had a reverse crown which you thought was 
> deliberate?  I would think that this board would be in tension.  I have trouble 
> imagining how you would build a board like this to be in compression unless the 
> ribs were on top.  Which side were the ribs on, by the way?  I think I remember 
> you saying that this piano had a very nice sound.  Did it have a lot of 
> cracks?

   >>>>> Yes I did post on this. It was a  Mehlin & sons and a Sohmer. Both 
in my recall had postive downbearing and intentionally built reversed crown 
boards. The Mehlin had a really dished board but had only one hairline crack at 
the tenor bridges end. The ribs in both cases were on the bottom. I like the 
sustaining qualities of these pianos.  I'm sure the performace of these differs 
from the conventional boards but it's a very atrractive sound. The lack of 
damage to the wood in tension I've seen doesn't support the cracking of 
assertions I and others have made in regards to tensioned panels but these are only two 
examples. By the way I just bought another sohmer 5ft 11" that has the same 
config. only it has aggraffes from the bottom of the tenor bridge up to the 
capo break. Regular bridge pins in the bass and top trebles.

          Regards ---Dale Erwin

>  
> 
> 
> 


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