Soundboard Pre-Stressing/Downbearing, was: Baldwin accu-hitch pins

[Farrell]

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Thank you very much for your response Dale. Some good info here. I will =
read those articles.

Terry Farrell =20
  ----- Original Message -----=20
  From: Erwinspiano@AOL.COM=20
  To: pianotech@ptg.org ; davehugh@msn.com=20
  Sent: Sunday, January 20, 2002 8:47 PM
  Subject: Re: Soundboard Pre-Stressing/Downbearing, was: Baldwin =
accu-hitch pins


  In a message dated 1/20/2002 6:41:51 AM Pacific Standard Time, =
mfarrel2@tampabay.rr.com writes:



      Terry


    >>>Welcome to the insomnia club. Considering all the variables in =
the always in flux soundboard system I've always thought that setting =
bearing to the degree you are speaking ofis a bit like making a jello =
foundation for a house. Just when you thought you had the right =
consistency poof it wiggled and the house fell over.
    Well it's not quite as bad as all that



    I have always been curious about the soundboard pre-stressing method =
of targeting downbearing, but when I ask how much to push the soundboard =
down, I get answers like: "enough", "not too much" and the like. The =
idea of getting a feel for how much the soundboard will deflect under =
load I should think would help target proper downbearing Now I see here =
we are using some numbers. This is a good thing. You say David Hughes =
deflects the board 3 mm at the center strut. You state that you drive =
wedges under each strut. How much do you deflect the board at each =
strut?


    >>>>The short answer is after driving the maple wedges in, readings =
of deflection will be appox.3 to 3.5 at the central strut about 2 ish on =
the next treble strut and 1 to.05 on the top. The lowest tenor strut can =
be forced lower than is needed and should not be over driven. It's value =
will be in the 2 mm range. But hey if you don't want to take my word for =
it e- mail Dave hughes at  davehugh@msn.com and ask him.
          Terry, save some time and read  John Hartman 'effects of =
downbearing on tone' aug,sept oct nov. 1996. Very good reference =
material. The last article will give you some numbers that I can see a =
person like you is ready to digest. I have posted a description of this =
in the last year on this list.



    Now the other question I have is that I assume the reason one would =
pre-stress the soundboard is to incorporate into your work how much the =
soundboard will deflect under load.



  >>>>>>..  Yes an approximation


     But if you deflect the soundboard 3mm or whatever, wherever, for =
any given=20

    soundboard, what does that do for you (i.e., you are pushing a stiff =
board 3mm down, and you would be pushing a highly flexible board 3mm =
down - under strung conditions, obviously the flexible board would =
deflect more under final strung downbearing)?

  .>>>>>> Ist question. Your making it stiffer and changing the =
impedance
  .>>>>>Yes but for the sake of discussion lets stick to newly crowned =
boards. The same principles can be modified for other situations. Rib =
crowned boards with either sugar pine or spruce ribs and duplicated to =
original rib heightswill result i a stiffer board than the old =
compression crowned boards and the amount they compress is fairly =
predictable.=20
    >>>>>>>> Remember jello? It moves. So does a new unstrung =
soundboard. Up And down with moisture content. So at what point in the =
moisture content cycle are you going to set your bridge =
heights/downbearing. More insomnia material.
    =20
       You could simply calculate the needed angles without =
pre-stressing the board.=20

    Now if you could pre-stress the board with downward pressure =
approximately equal to that of the final string downbearing in the =
various regions of the scale, then we would be truly simulating the =
soundboard's reaction to the strung-up load and accurately calculate our =
desired string downbearing angles, etc. Of course to do this we would =
need some sort of pressure gauge/device.

    >> Terry I know Steinway doesn't carry much weight with you but =
they've done it this way for a hundred fifty years but so has mason and =
others

  >>>See Bob Hofs article on this last year he uses go bars off the =
Ceiling. The down side to that is I can't feel what go bars are doing  =
in the same way as I can with my fist.
    >> With the maple wedges I insert them between the struts with the =
left hand as I  pound with my right fist on the bridge top. As the board =
sinks to appox. 3 mm it gets so stiff it feels like your pounding on a =
gym floor. Or you pound till it feels real stiff It's an aquired feel =
that's not difficult to get the hang of. Repeat at each strut.=20


    What about a rig similar to a pneumatic rib clamp. Just use the half =
with the fire hose. Clamp it to the rim above the bridge, inflate, and =
there you go with measurable pressure applied all along the long bridge =
in much the same manner as downbearing would be once the piano is =
strung. You could also use four segments of fire hose to apply =
appropriate variable pressure to each of the four sections (and one on =
the bass if you want to do that). The result would be a board stressed =
in very much the same manner as it would be after stringing. Then you =
could set your downbearing exactly where you want it, armed with the =
knowledge of exactly (or there abouts) how much the soundboard will =
deflect!


     >>>>Dave Love was right, no access to the bridge.



    So now, readers..........you are thinking one of two things: Either =
Terry Farrell needs to get his head screw on properly, i.e. it's a dumb =
idea, or it sounds like a great idea, now go try it Terry and write us =
back. So what do you think?


     >>>Your doing some good thinking. no worrys you're on your way


    Maybe it is the case that with a given soundboard construction, the =
boards are consistent enough to yield predictable result when =
pre-stressed 3mm or whatever. Is this the case, and thus would yield a =
more accurate pre-stressing a waste of time?


      >>>>This what  I'm gettin at


    I guess I'm just thinking and probing for information. Any input =
would be appreciated. I realize the thing to do is put in a couple dozen =
boards, try some different methods, and see what happens.

  >>> This helps a lot. Of course  don't forget the fun factor. I hope =
this is helpful verses annoying.
  But boy oh boy, that will be years, and I would fear, some tears. I'm =
just trying to=20

    arm myself with as much info a possible before I screw up too darn =
many pianos.

    Terry Farrell

    SNIP




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