laminated ribs

[Joe And Penny Goss]

Hi Ron,
Your I beam rib really makes sense to me.
The strongest and yet lightest at the same time allowing the board its
fullest range of vibration.
In the post below you mention educated guesses as to the final shape of the
board after the string load has been added.
Am I correct in my assumption that since the treble ribs are shorter, they
will be stiffer and allow less compression, while as one moves to the tenor
there will be more movement in the profile of the board?
Also little movement in the bass due to the placement of the bridge?
Would one gather insight into the loading of the bridge using say a 50 or
100 lb weight on each rib at the point that the bridge would be placed with
the rib suspended near its end?
You got me thinking and thats a dangerous thing <g>
Joe Goss RPT
Mother Goose Tools
imatunr@srvinet.com
www.mothergoosetools.com
----- Original Message ----- 
From: "Overs Pianos" <sec@overspianos.com.au>
To: "Pianotech List" <pianotech@ptg.org>
Sent: Sunday, February 19, 2006 4:15 PM
Subject: Re: laminated ribs


> Richard,
>
> The downbearing (vector) force on the sound board
> is equal to the SIN of the angle of deflection
> times the string tension.
>
> If there was absolutely no down bearing angle, it
> follows that there would be no downbearing force.
> The SIN of zero is zero so the string tension
> vector component force would be zero.
>
> If the down bearing angle was 90 degrees, with
> the speaking length segment parallel to the board
> and the back scale heading vertically downwards,
> the down bearing force would be equal to the
> string tension, ie. the speaking length segment
> would be contributing nothing to the down bearing
> force, while the back scale segment would be
> contributing its full string tension. The SIN of
> 90 equals 1.0. String tension X 1.0 equals string
> tension. You can see how it all works.
>
> So if you have 160 lbs unison string tension with
> a downbearing angle of 2 degrees, the downbearing
> vector force for this unison string would be;
>
> Downbearing = 160*Sin2.0
>
> Downbearing =5.583 lbs
>
> The downbearing force for the whole note would be
> 3 X 5.583 if the note was a trichord, at 16.75 lb.
>
> If you are using an excel spreadsheet for your
> calculations, remember that the downbearing angle
> will need to be converted to radians.
>
> Yes, there is a large variation in what people
> believe is an appropriate level of downbearing.
> If you measure a few pianos around the place
> you'll find that there is a lot of variation in
> the downbearing angle also.
>
> The 2 degree figure you quoted I would consider
> to be too high for a real world piano.
> Bösendorfer have typically set their pianos with
> angles approaching 2 degrees strung. This is a
> little higher than I would feel comfortable with.
> When Ron N was here a couple of years ago we
> looked at our no. 5 with a Lowel gauge and it
> measured almost right on 1.3 degrees over the
> whole piano. This yields a total downbearing
> force on our no. 5 of 427 Kg (941 lb). I wouldn't
> recommend these figures for an older or weaker
> panel but it works just fine for our I-rib
> design. Setting the downbearing angle is a
> balancing act between how much the board will
> sink and how much force we wish to apply.
>
> When looking at a given piano, I suggest that you
> set up a spreadsheet to calculate the downbearing
> force you are planning to set up per rib. Note
> also that setting an unstrung angle of say 1.5
> degrees won't result in a downbearing force of
> tension X SIN(1.5). Its the resultant string
> deflection angle when the piano is at pitch and
> the board has stabilised (sunken to equilibrium)
> under load which will determine the actual
> downbearing force. So you need to make an
> educated prediction on how much a board will sink
> under tension to get an idea of the resultant
> downbearing force.
>
> A common scenario with new pianos is for techs to
> measure a down bearing figure which on the face
> of it looks OK, but very often the sound board
> has sunken to a state where it is pushed almost
> completely flat by the down bearing angle which
> was set into the piano. In these instances the
> board is too weak for downbearing loads which are
> being applied or the unstrung angle wasn't set
> properly. Either the downbearing unstrung angle
> should be reduced or the board strengthened to
> withstand the setting angles to which it is being
> asked to resist. So often technicians will look
> at a sound board and declare that it is fine
> because the downbearing angle measures some
> wonderful figure. But if the board has been
> pushed inside out before the customer's ink is
> dry on the cheque, things ain't too good,
> regardless of what the downbearing gauge might
> indicate.
>
> Get an accurate downbearing gauge and a thread
> length for looking at crown, and measure a few
> pianos old and new. You'll develop a picture of
> what's happening.
>
> Ron O.
>
> >Please correct if this is entirely wrong... but
> >I thought that since the string was being
> >measured in terms of its tension (pounds)  one
> >could simply the problem  as a like sided
> >triangle with half the pounds on each leg. Since
> >the measurement is taken in the deflected
> >condition... you have basically the hypotenus
> >and all angels of a right angle triangle
> >available to figure the amound of deflection..
> >pounds in this case.  So 160 pounds with a 2
> >degree deflection at the bridge  yields
> >
> >Sin 1 x 80  = 1.396192515  lbs downbearing,
> >which is 1.745 % of the string tension.
> >
> >er... yes ??
> >
> >RicB
> >
> >
> >-------------
> >>    So knowing all of the above, what is the equation that will
calculate
> >>  an approximate string bearing load under the conditions I describe?
> >
> >Beats me. I use the SIN(RADIANS(degree measurement))*tension
> >per unison, and add them up in my spreadsheet.
> >_______________________________________________
> >Pianotech list info: https://www.moypiano.com/resources/#archives
>
>
> -- 
> OVERS PIANOS - SYDNEY
>     Grand Piano Manufacturers
> _______________________
>
> Web http://overspianos.com.au
> mailto:ron@overspianos.com.au
> _______________________
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