Folks,
Sorry I wasted bandwidth. This was an
old question from David Love that I saved. Looks
like I found it again by accident and this time
decided to respond. Please disregard.
Greg
At 06:17 PM 3/7/2006, you wrote:
>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
> >_______________________
> >_______________________________________________
> >Pianotech list info: https://www.moypiano.com/resources/#archives
> >
> >
> >_______________________________________________
> >Pianotech list info: https://www.moypiano.com/resources/#archives
>
>Greg Newell
>Greg's piano Forté
>mailto:gnewell at ameritech.net
>
>
>_______________________________________________
>Pianotech list info: https://www.moypiano.com/resources/#archives
Greg Newell
Greg's piano Forté
mailto:gnewell at ameritech.net
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