laminated ribs

[Joe And Penny Goss]

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Hi Dale,
An uneducated guess here. Are you also reducing the size of the rib =
some, seeing that it is also stronger?
Joe Goss RPT
Mother Goose Tools
imatunr@srvinet.com
www.mothergoosetools.com
  ----- Original Message -----=20
  From: Erwinspiano@aol.com=20
  To: pianotech@ptg.org=20
  Sent: Saturday, February 18, 2006 9:04 PM
  Subject: Re: laminated ribs


    Ok, now this is an interesting discussion.  Admittedly not being a =
math guy I'm still interested in putting some numbers on some scales of =
things I've seen as a bench marks for comparison.
     Let's just take one case I have  first hand knowledge of.  I =
rebuilt & 1960 Stwy L 3 years ago that lived in a Fresno area church =
from the beginning of it's creation to the present so it has survived =
wonderfully well.
    I was keenly impressed  by the balance of sound, both in power &  =
sustain.  I measured the bearing with a lowell gauge & though I don't =
have numbers any more to give you my recall is that the top capo had =
over 2 degrees of deflection & the 2nd capo  about 2 or more &  the =
middle was  1 1/2 degrees tapering down to 1/2  in the bottom & the bass =
had positive but minimum bearing as it should be with a cantalever.  The =
crown string stretched across the boards underside revealed lots of =
residual crown in the strung condition & more than any other C.C. board =
I've ever seen up to that time.  All that to say it was in my opinion a =
text book Steinway/belly  set up both in terms of crown & bearing.   =
These are IMO the kinds of observations that  are important to make when =
we find something that is working really well.=20
    The Stwy L scale as I recall has an average treble tension at 160 =
lbs per string. It is obvious to see that the majority of the bearing =
pressure on the long bridge is increasing  gradually the higher up the =
scale we go.=20
    So knowing all of the above, what is the equation that will =
calculate an approximate string bearing load under the conditions I =
describe?
    If it's the one- 40th rule for simplicity then  40 divided into  160 =
strings  equals 4 pounds per string. Let's remove most of the bass =
strings from this equation for now, since theoretically there isn't much =
bearing there & we have approx. 160 strings times 4 pounds equals 720 =
lbs. add in say 80 lbs for the bass & it's about 800 total pounds give =
or take
       There is a much more accurate & glamorous formula for this but I =
dont' have it at my finger tips.  If the scale tension averages 180 lbs =
per string then we're talking 4 1/2 pounds per string which bumps total =
bearing load up another 100 ish pounds.
    My point in all this is that if we are using stronger engineering =
materials & principles which building better stronger rib structure, =
which we are, then surely our rib crowned & supported boards will =
survive as well & IMO longer than this example of a C.C Steinway L  I =
cited above=20
     Don't you think?
    Dale Erwin

    Consider a basic scale of moderately high tension. Say 40,000 lbs. =
overall. With this string tension 1,000 lbs of string down force equals =
2.5% of scale tension. That is quite a lot considering that most =
companies are claiming string down force more on the order of 0.5% to =
1.5% of string tension (which would be 200 to 600 lbs). I thought I was =
setting my initial string down force pretty high at around 1.0 to 1.5%. =
I don't like thinking about what I'd be doing to a board loading it up =
to 2.5%. I can't imagine it being happy enough at that level to want to =
stay there.

    Del

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