laminated ribs

[Erwinspiano@aol.com]

---------------------- multipart/alternative attachment
 
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. 
  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. 
  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 
   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



 

---------------------- multipart/alternative attachment
An HTML attachment was scrubbed...
URL: https://www.moypiano.com/ptg/pianotech.php/attachments/69/d9/c9/5c/attachment.htm

---------------------- multipart/alternative attachment--