Hamlet Davis bridge agraffes/ Reverse crown Steck

[DALE ERWIN]

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 Hi Phil
 =20
   Comments interspersed below
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----- Original Message -----
From: Phillip L Ford
Sent: Wednesday, November 21, 2001 10:46 AM
To: pianotech@ptg.org
Subject: Re: Hamlet Davis bridge agraffes/ Reverse crown Steck
 =20
Dale,
The reverse crowned board with downward bearing
is an interesting idea. If you installed the crowned
board into the piano without ending up with compression
preload then the downbearing should preload the board
in tension to approximately the same degree as it would
preload the board in compression in the more usual
arrangement.
Phil. ------ I,m not sure about that.  Perhaps the deflection would be th=
e same say 3 or more millimeters under string presurre but here's a thoug=
ht.  With an upward crowned board the pressure of downbearing does stiffe=
n it but to my mind doing so  compresses the wood molecules into a smalle=
r space thereby having a constricting effect.I.E compression cracks & rid=
ges.   In the reverse crown model just the opposite is true. The wood mol=
ecules are being tesioned or pushed apart. Not just the panel but also th=
e ribs. Is not the net effect the same as creating a reverse coil spring.=
  Seems to me the first model is compressing a spring and the other one s=
tretching a spring. Whatever it is there is something very different abou=
t this system.
     As I see it the purpose of the crowning
and preload due to downbearing is to stiffen the soundboard
system without adding mass (as you could do say by
thickening the board or adding heavier ribs). =20
Phil--true this is the theory and in practice balancing the impedance cha=
racteristicsthis is what rebuilders  do but that idea has its limits. Obv=
iously  too much impedance at some point provides diminishing returns. Ho=
w much mass and stiffness is is enough are questions I ponder freequently=
.
Where you
need this is at the top end, hence the substantial
downbearing at the top end, with almost none at the bottom
end of the scale. =20
Phil---In a pre-stressed downbearing setup even the bottom of the scale h=
as some fairly significant pressure,but you are correct that proportional=
y the trebles have a greater down bearing force
  It would seem to me that the stiffening
due to crowning or arching the board would be the same
whether it were crowned up or down.  It also seems to me
that it wouldn't matter whether the stiffening from the
downbearing preload were a tension or compression preload.
It also seems that you would get the added benefit that as
the board tended to take on any set from the preload with
the reverse crown arrangement the set would tend to give the
board more crown rather than flatten it out as in the more
usual arrangement. =20
Phil- Surely this is true when considering the wood expanding and drawing=
 into further tension with humidity However I'm not sure this design woul=
d require as much panel drying before rib glue up(6% E.M.C.or so) so comp=
ression set may not be as much of the equation. Although I believe it wou=
ld still require this reasonable amount so that it would resist cracking =
over time because of its tension loading. When I set up the bearing on a =
6 ft. Julius bower with reverse crowning I did not push it as hard as I w=
ould a positive crowned board as It sang like an angel with modest downbe=
aring before it was torn down for stringing and bridge caps etc. I didn't=
 see any signs of compression set.
 What would concern me is that wood tends
to have much less tension strength than compression strength
across the grain and would therefore crack or take on
permanent set more readily with a tension preload than
with a compression
preload.  However, wood has much more tension strength than
compression strength along the grain and perhaps this would
help.  It's interesting that on the reverse crowned boards
you have seen you haven't noticed deterioration due to the
tension preload.
Phil- It is very interesting indeed. The strength along the grain as you =
say must be a factor. One would expect a great deal of cracking but the B=
ower had only a couple very small hairlines right at the bridge end (note=
 27) which were easily hand shimmed for cosmetics. Really this board look=
ed like a salad bowl. My last comment is that this design is intriguing b=
ecause the sustain indicates quite excellant power with and incredibly lo=
ng ,free and unrestricted sounding sustain thru out the scale. Considerin=
g recent discussions it's interesting  that the top trebles had hard and =
quite pointy triangular bearing bars for front terminations. They were he=
ld in  place by string pressure. Oh yeah longish backscales too. As I sta=
ted before I've encountered this design now by a variety of makers and ea=
ch exhibited a sustain/free sound that imediately grabbed my attention an=
d the boards were not split up. Veeery interesting eehh. or at least much=
 food for thought.
May you all have a wonderful thanksgiving. We are richly blessed!
---
   Best----- Dale Erwin


On Sun, 18 Nov 2001 06:48:16
DALE ERWIN wrote:
>Tom and list
>
>    You know I have been following this thread and it is interesting to =
me how many makers tried out this idea. I rebuilt a George Steck 6 ft. gr=
and last year with this arrangement. When I first encountered the piano t=
he sound was absolutely magic. The sustain just floated for what seemed l=
ike forever . The more interesting thing was that the board was a reverse=
 crowned board with a positive set downbearing. I mean bearing set pressi=
ng in the usual direction.
>   A bit more time consuming to string but what a gem. I keep finding al=
l kinds of sound board conditions and configurations which aren't suppose=
d to work that do and others that should that don't!
>  I have to figure that the weight of the Big  Brass agraffes also contr=
ibuted to the impedance properties of the board probably a sustain enhanc=
ing dynamic.
>
>       Ain't life interestin---Dale Erwin----- -----

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