[pianotech] Action ration measuring

[Jim Ialeggio]

Ron said:

<It always annoyed me that the cumulative leverage of the presumed 
moment arms matched so poorly with Dale's 6mm in/out direct measurement. 
Something's wrong, and I'll take direct first.

I've tried to press the point that though these formulas assume the 
pivots are true pivots, they are not all true pivots. This has to effect 
the equivalence of the equation. This is especially and critically true 
of the pivot at the knuckle/jack contact.  The actual ratio is variable. 
Hohf calculated a 6.125 advantage of the hammershank @ rest modulating 
to a 8.02 advantage @ letoff on one of his models. The argument then is 
presented that we "solve" for that complicating fact by measuring the 
levers @ half stroke.

Okay...take the measurements @ half stroke. However, if one calculates 
from half stroke, we are faced with the question of what precisely the 
definition of half stroke is.  I suggest, that the actual location of 
half stroke makes a difference in whether the resulting ratio agrees 
with real-world regulating specs.

My take...the position of half stroke should be determined by 1st 
determining the  full arc the hammer would travel, driven by the key at 
its full dip,  with the hammer unimpeded by the letoff button. Full 
hammer  travel in this scenario is of course more than blow(-)letoff, 
but it exceeds blow as well.

This works the ratio question backwards from the result to the specifics.

(Full hammer travel)/(overall key dip) = overall ratio.
(Full hammer travel)/2 = hammer @ half stroke.
(Full key travel)/2 = key @ half stroke

Take lever arms from this position, with the critical knuckle/jack 
contact and resulting lever arm being at the actual contact between jack 
tip and knuckle radius @ half stroke.


(the thread title seems to think the ratios should be rationed, and I 
would absolutely agree...in the strongest of terms...if only I could 
figure out what that means...it seems like it ought'a be a good thing to 
agree with...<insert absurd emoticon here>)

Jim Ialeggio

-- 
Jim Ialeggio	
jim at grandpianosolutions.com
978 425-9026
Shirley Center, MA