---------------------- multipart/alternative attachment =20 In a message dated 2/19/2006 3:15:56 P.M. Pacific Standard Time, =20 sec@overspianos.com.au writes: Richard, The downbearing (vector) force on the sound board =20 is equal to the SIN of the angle of deflection=20 times the string tension. If there was absolutely no down bearing angle, it=20 follows that there would be no downbearing force.=20 The SIN of zero is zero so the string tension=20 vector component force would be zero. If the down bearing angle was 90 degrees, with=20 the speaking length segment parallel to the board=20 and the back scale heading vertically downwards,=20 the down bearing force would be equal to the=20 string tension, ie. the speaking length segment=20 would be contributing nothing to the down bearing =20 force, while the back scale segment would be=20 contributing its full string tension. The SIN of=20 90 equals 1.0. String tension X 1.0 equals string=20 tension. You can see how it all works. So if you have 160 lbs unison string tension with=20 a downbearing angle of 2 degrees, the downbearing=20 vector force for this unison string would be; Downbearing =3D 160*Sin2.0 Downbearing =3D5.583 lbs The downbearing force for the whole note would be=20 3 X 5.583 if the note was a trichord, at 16.75 lb. If you are using an excel spreadsheet for your=20 calculations, remember that the downbearing angle =20 will need to be converted to radians. Yes, there is a large variation in what people=20 believe is an appropriate level of downbearing. =20 If you measure a few pianos around the place=20 you'll find that there is a lot of variation in=20 the downbearing angle also. The 2 degree figure you quoted I would consider=20 to be too high for a real world piano. =20 B=F6sendorfer have typically set their pianos with=20 angles approaching 2 degrees strung. This is a=20 little higher than I would feel comfortable with.=20 When Ron N was here a couple of years ago we=20 looked at our no. 5 with a Lowel gauge and it=20 measured almost right on 1.3 degrees over the =20 whole piano. This yields a total downbearing=20 force on our no. 5 of 427 Kg (941 lb). I wouldn't=20 recommend these figures for an older or weaker =20 panel but it works just fine for our I-rib=20 design. Setting the downbearing angle is a=20 balancing act between how much the board will =20 sink and how much force we wish to apply. When looking at a given piano, I suggest that you=20 set up a spreadsheet to calculate the downbearing=20 force you are planning to set up per rib. Note=20 also that setting an unstrung angle of say 1.5=20 degrees won't result in a downbearing force of=20 tension X SIN(1.5). Its the resultant string=20 deflection angle when the piano is at pitch and=20 the board has stabilised (sunken to equilibrium)=20 under load which will determine the actual=20 downbearing force. So you need to make an=20 educated prediction on how much a board will sink=20 under tension to get an idea of the resultant=20 downbearing force. A common scenario with new pianos is for techs to=20 measure a down bearing figure which on the face=20 of it looks OK, but very often the sound board=20 has sunken to a state where it is pushed almost=20 completely flat by the down bearing angle which=20 was set into the piano. In these instances the=20 board is too weak for downbearing loads which are=20 being applied or the unstrung angle wasn't set=20 properly. Either the downbearing unstrung angle=20 should be reduced or the board strengthened to =20 withstand the setting angles to which it is being=20 asked to resist. So often technicians will look=20 at a sound board and declare that it is fine =20 because the downbearing angle measures some=20 wonderful figure. But if the board has been=20 pushed inside out before the customer's ink is=20 dry on the cheque, things ain't too good,=20 regardless of what the downbearing gauge might=20 indicate. Get an accurate downbearing gauge and a thread=20 length for looking at crown, and measure a few=20 pianos old and new. You'll develop a picture of=20 what's happening. Ron O =20 ---------------------- multipart/alternative attachment An HTML attachment was scrubbed... URL: https://www.moypiano.com/ptg/pianotech.php/attachments/66/dc/e6/64/attachment.htm ---------------------- multipart/alternative attachment--
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