Action Geometry

[Richard Brekne]

Hi List

Got to fooling around abit with my Petrof Grand today and quickly decided
that rather then start taking a bunch of measurements ala Stanwood style, I
would be better off getting better aquainted with what happens when action
spread is changed. It became obvioius from the get go that I needed to
understand this better before Stanwood stuff was going to do me a whole lot
of good.

The first things I noticed  results of increasing the spread. I had
expected to have to lower the drop screw a bit, but found that this really
didnt get very much affected. Right away I noticed that the area of the
whippen that contacts the letoff button is pretty flat and close to
perpendicular to the drop screw. (I suppose this can vary from action to
action), so moveing the whippen wasnt going to require much adjustment as
in the case of letoff where the comparably large angle of the repetition
lever raises both the hammer rest position and the letoff quite a bit.

Also easily noticed is the difference in touch weight. Increasing the
spread by just 1 millimeter caused a very noticeable lightening of the
touch weight.

I got to thinking about the spread and the so called magic line. Putting
into some perspective posts I have recently read about capstans having to
be moved (sometimes quite a bit) and it struck me there is something a bit
undependable about this magic line. My reasoning is as follows.

Given proper jack to knuckle position when the hammer is at proper rest
position (hammer to string distance is correct), then  when the whippen is
raised to bring the hammer to drop position,  the jack should just be clear
of the knuckle and well away from the stop felt. Another thing that needs
to be the case is that when drop and letoff distances are correct then the
drop screw and the letoff button each come into play at exactly the same
time. There seems to be only one spread that allows for the closest
allignment to these two conditions. To much or too little spread seems to
affect the distance between letoff and drop when both touch there
respective actuaters at the same time.

This leads me to believe that spread is actually a function of the
relationship between the hammer shank assembly and the whippen, and really
quite independent of the capstan as such. If the capstan is not on this
"magic line" when this relationship is correct, the key dip is correct, and
the capstan height is correct for aftertouch, then the capstan is not
positioned correctly along the key and should be moved accordingly.

Ok... pick me apart...grin.. Gots to get this clearer in my mind for sure.

--
Richard Brekne
RPT, N.P.T.F.
Bergen, Norway