[pianotech] action ratio

[David Love]

I don't quite think it's apples to oranges because for any given action
ratio there will be at least a limitation to the regulation specs or the
dip/blow relationship.  I agree that the formula is confusing but I don't
think it has to be.  Determining when let-off and aftertouch actually start
seems to be the problem.  Once established, however, I see no reason why a
default setting for the sake of this formula can't be used.  Moreover, I
find that it's the aftertouch part that is confusing.  As far as the formula
goes, my own experience suggests that a let-off number of 1.5 and an
aftertouch number of 2.5 produces regulation specs that I expect to find
with a given action ratio.  The problem seems to be defining exactly what
aftertouch is, at least as it pertains to this formula.  Blow, dip and
let-off are more easily defined.   

David Love
www.davidlovepianos.com


-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Jim Ialeggio
Sent: Sunday, August 26, 2012 8:32 AM
To: pianotech at ptg.org
Subject: [pianotech] action ratio

David L said:

<The question at hand has to do with what exactly is the after touch spec
that's used to make the formula ratio by product of the levers equal to the
blow/dip ratio as given.
(and)
<But when I input those numbers in this calculator, as you can see, then by
that formula a 5.75 AR with 45 mm of blow distance should regulate with 8.6
mm of dip.

This illustrates the confusion I find in these formulas, if they are to be
used from a design perspective rather than as a ballpark reactive position
in the field. I feel the (blow distance-let off)/key
dip-aftertouch) part of the equation, when compared to the lever arms part
of the equation is apples to oranges.

The lever arm side of the equation looks at the lever arms as the product of
fixed ratios.  However, the blow distance ratio does not look at blow
distance and key dip as fixed ratios, but rather inputs arbitrary
letoff/aftertouch as necessary to make that side of the equation match the
fixed ratio side, as I think you've noted, David.  Or seen another way, if
you are going to use the blow distance side of the equation as given, then
the lever arm side must take into consideration the whip's changing leverage
and the shank's changing leverage during letoff. This changing leverage
presents difficulties that would require computing complexities way beyond
simple algebra.

Taken in this light, your shy 8.6mm dip could be more a result of blow being
quantified artificially.  According to the fixed leverages, blow is not the
distance between hammer at rest and the strings minus letoff, but rather
(Full unimpeded travel of the hammer with out any letoff or
strings) = Action ratio*(key dip-aftertouch).

By the way, my comment here is not that this formula doesn't empirically
work, but rather that the thinking behind it seems to be selective and thus
confusing, at least to me.

Jim Ialeggio




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