Action Analyses (was Capstan Relocation)

[Bill Ballard]

At 10:46 PM +0200 10/22/00, Richard Brekne wrote:
>Bill Ballard wrote:
>
>  > And speaking of "ya ever look close there", did you ever watch a jack
>  > get momentarily pulled backwards (squashing the jack button felt)
>  > which the cycle first starts up?
>
>Yepp.. noticed this right away dinking around with the spread on my 
>action model.
>Also notice that when the spread is just barely on the wide side 
>(jack center on
>the perpendicular to hammershank at the knuckle with the hammer at rest) this
>didnt happen. That seemed to be because the jack looked more like it 
>was behaving
>like a pivot during this portion of the keystroke. Instead of being dragged by
>the knuckle,,, it rocked nicely on the knuckle while both  pieces were moving
>through their respective arcs.

It is ever so momentary. I've always assumed that the sliding 
friction between the top of the jack and the knuckle had a static 
phase, one which at the start locked the jack top to the knuckle, and 
which was not transformed into a sliding friction until the point 
where the potential energy store in the compression of the jack 
adjusting button felt finally exceeded the force of static friction 
at this spot. Remember that while the contact point on both arcs (the 
knuckle's and the jack top's) are both heading in the same direction, 
the horizontal component (sliding motion) is greater in the knuckle's 
arc, and it is also increasing. No wonder the jack is not inclined 
(no pun intended) to immediately break free of this static friction 
grip with the knuckle.

I'm not sure that this is the explanation for the effect, or whether 
your observation regarding the spread contains the answer. It would 
seem quite likely that this effect could still occur even in an 
action in which axes and contact points had been properly dealt with, 
such as in Ron's action.

How modest of Ron not to respond when I asked if he had anything to 
tell us about his action. Seriously! He'd given a full description of 
it in a thread I'd missed last month.

Bill Ballard RPT
NH Chapter PTG

"No one builds the *perfect* piano, you can only remove the obstacles 
to that perfection during the building."
     ...........LaRoy Edwards, Yamaha International Corp