action ratios

[Richard Brekne]

David Love wrote:

> If you're asking me to posit that as a general rule, I'm not ready to do
> that.  That has been the nature of my recent inquiries.  I can only say that
> at this time that for this piano that was the case.  That there is a
> relationship between the action ratio and how the action regulates shouldn't
> come as any real surprise.

Of course not, the fact that lower ratio means in some general sense either less
blow, more dip, earlier letoff / drop or some combination of these is old news.
That a 5.3 ratio should be tied to an impossiblity to regulate
satisfactorilly...even accepting a strict 10mm dip is new... at least to me.


> Whether or not there are other mitigating
> factors is part of the question.

I would think a big part. Lets think this one through a bit as its kind of
critical. You might remember I asked a few weeks back about how far you could go
at a 10 mm dip in relation to ratio and blow.

Lets me start off with a simple approach and see where that takes us. 5.3 ratio,
10mm dip. That means 53 mm hammer travel taking letoff/drop out of the picture
for a moment. 44 mm blow and 1.5 mm letoff gives us 10.5 mm hammer travel to
work with for letoff / jack escapment... or translated back into key travel we
have 1.98 mm key travel... lets say 2 mm key travel at the front to get
aftertouch to work right. Seems doable.

Given a .50 Key ratio we get 1 mm capstan travel which translates into at least
2 mm jack tail travel. The whippen I have in front of me has a 3.3 length ratio
between the two arms. Rougly translates into  3.3 mm long arm travel for 1 mm
short arm travel, or about 6.6 mm travel at the jack top,  just over half the
diameter of a 12 mm knuckle plus .5mm to take in the entire core width. This is
jack clearance...but just barely. Adding just 0.2 mm dip (hardly a draconian
measure) gets us 7.26 mm jack top travel.  Or given a key ratio of 5.2  we get
6.9 mm jack top travel.

Now of course all of this is pretty rough, but I tried to figure with
conservative lengths. Point of all this is that jack escapement seems to me to
be somewhat less then directly related to overall ratio. Key ratio seems
directly more important, as does the ratio of the two arms of the jack itself.
And the whippen center / heel / jack tail  relationship as well. And theres
probably a good deal more to play around with.

I will buy that things start becoming interesting at 5.3. But then thats what I
have been saying all along. And I suppose the same holds true in the opposite
sense for around 6.6.

Cheers

RicB

--
Richard Brekne
RPT, N.P.T.F.
UiB, Bergen, Norway
mailto:rbrekne@broadpark.no
http://home.broadpark.no/~rbrekne/ricmain.html