laminated ribs

[Greg Newell]

David and others,
         I understand your questions and they are 
good ones. Perhaps my answer is simpler than 
anyone is looking for but ........ The board is 
designed in such a way as to be in direct 
opposition to the downward force of the 
individual and combined force of the strings. 
There isn't any magical relationship that I'm 
aware of as a carryover from a CC setup save that 
the opposing spring of the board is still in 
direct opposition to the strings. It seems to me 
that the beauty of the RC&S system is the 
achievable predictability of the result not 
necessarily the added potential in it's strength 
capabilities. Just because the capability exists 
does not necessarily mean it is exploited.
         Consider that in many cases where a RC&S 
system is used it is also coupled with the other 
design elements of an adjustable plate support 
system and a roll pin type arrangement both of 
which assist in setting a micro-adjustable down 
bearing.  This is a real part of the beauty of 
this overall plan. Taken alone these two would 
serve to provide a much better result even in a 
CC board system since it is so fully adjustable. 
The preference for RC&S in my mind seems to be 
one of longevity. Since the compression / tension 
relationship is mostly or wholly residing in the 
ribs the panel is far less likely to crack over 
time resulting  in a much happier customer in the 
long run not to mention the possible elimination 
of the killer octave scenarios and bridge roll 
scenarios that we all know and love.
         Please know that I am somewhat a 
neophyte in this and my opinions mean very little 
if anything at all. This is all just my current 
take on the subject and hey, you asked!

best,
Greg


At 11:03 AM 2/22/2006, you wrote:
>There is another issue to be raised.  How should one match the scale
>tensions and anticipated downbearing angles to the rib scale.  There are
>choices to be made.  I presume that you want a certain amount of deflection
>of the soundboard assembly and that given a certain scale with a certain
>downbearing load, you can calculate the panel assembly stiffness and preset
>crown (in and RC&S board) to achieve that amount of deflection.  But there
>are yet various ways to achieve that amount of deflection.  For a given
>assembly you could increase the scale tension and lower the downbearing
>angle or decrease the scale tension and increase the downbearing angle.  You
>can design an assembly with greater stiffness to go with a lower scale and
>greater downbearing or a lower stiffness to go with a higher scale and less
>downbearing, for example.  Each combination, I presume, will produce its own
>unique tonal characteristics and, probably, require a hammer of different
>density and/or mass.  Those of you who are designing boards, how would you
>characterize your goals and why?.  If we can produce a RC&S board that will
>be able to accommodate any particular variation in load, what is so magical
>about the .5  - 1.5 degrees of downbearing that seems like it came about
>mostly due to the limitations of compression crowning.  Further, in an RC&S
>board, what combination is most likely to give the general tonal
>characteristics of your successful CC board.  And let's allow ourselves to
>speculate even if we haven't actually built each variation.
>
>David Love
>davidlovepianos at comcast.net
>
>-----Original Message-----
>From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
>Of Overs Pianos
>Sent: Sunday, February 19, 2006 3:15 PM
>To: Pianotech List
>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 at overspianos.com.au
>_______________________
>_______________________________________________
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>
>
>_______________________________________________
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Greg Newell
Greg's piano Forté
mailto:gnewell at ameritech.net