S & S 'A' parts, geometry -- shanks and flanges

[Richard Moody]

Newton, 
	I have heard about this line running from the key balance point to the
whippen c pin, and the capstan bisects it ... is it at rest or half way
through?..., but I have never really understood why it needs to be that
way. But what can a technician really do about it?. Capstans have to be
turned up to set and level the hammer line, due to knuckles compacting,
back rail felt compacting, and hammer filing. (I would be surprised if
they got turned up or down more than 2 mm, so there is really not much
varation in the line once the action is constructed).
	A gear can be considered a series of levers. Of course the piano action
is a
series of levers. As such they move in circles.  The hammer at  45 mm
from the string travels further, because it goes in an arc.  But I would
think it is still proportional.  However I would think the vector forces
would be constantly changing. 
Fortunately we don't have to worry about that. Execpt perhaps with the
jack
and knuckle.  
	I would guess that at some point in their happy travels, the jack should
be at 180 degrees to the diameter of the knuckle. (To take advantage of
maximum vector forces) (When this is of maximum advantage, it might be
interesting to ask a mechanical engineer.)  I believe this can be gotten
sooner by positioning
the jack a little closer to the flange, ie more to the front side of the
wood core.  However this
might give a more chunky feel to the let off.  
I am used to the opposite, even a
tweek further than "the back edge of the jack in alingmnet with the back
edge of the wood core" because it seems a better feeling after touch.  

If 9.7 diameter is optimal I would guess knuckles would be made at 10mm
because
they will compact. 

Richard Moody 	

----------
> From: Newton Hunt <nhunt@jagat.com>
> To: pianotech@ptg.org
> Subject: Re: S & S 'A' parts, geometry -- shanks and flanges
> Date: Monday, October 05, 1998 9:38 PM
> 
> Ok, Richard, good reasoning but let's take it a little further.
> 
> By increasing the size of the knuckle (your numbers are good) the
> capstan has to be turned down thereby taking it off the optimum line
> between key balance point and wippen center pin.  This will increase
> force required to depress the key.
> 
....................................................<> 
> Therefore the size of the knuckle is actually optimized to LESS than 10
> mm, somewhere around 9.7 mm or there abouts.  This was determined by
> mathematically determining what is the smallest number of teeth an
> involute teeth a gear can have (13) and the optimum diameter of the
> tooth radii, therefore it's diameter of about .3675" (if my memory
> serves right).
> 
> That is why geometry is so critical to action performance and why so
> much attention _must_ be paid to action part replacement selection.
> 
>                 Newton
> 
>