Escapement friction (was Re: Min LO)

[Phillip Ford]

>At 9:45 AM -0700 6/7/03, Delwin D Fandrich wrote:
>>This is the part that intrigues me. The ability to set very close letoff 

>>and the ability to control the action at pianissimo levels.
>
>At 7:46 AM -0700 6/8/03, Delwin D Fandrich wrote:
>>So, when I hear some say some grand action design can be regulated with 
>>let-off right up to within 1 mm of the string line I at least want to 
>>give the thing some thought. And one of those thought is, "I wonder how 
>>this thing would work with a nice graphite-filled ABS or Nylon jack?"
>
>Are you perhaps thinking of the final turnstile the hammer has to go 
>through to make it to the string, the friction barrier of escapement? At 
>the initial turnstile (DW, or in a non-friction world, BW), nothing 
>happens until you put the correct number of coins in the slot. At 
>escapement, with the hammer's momentum, at least you get a consolation 
>prize for being a dollar short: a "Viennese escapement". As in the 
>Prell-mechanik, the rep lifts the hammer until the jack tender hits the
LO 
>button,  and because the driving force at the key is less than the 
>escapement friction, everything stops dead in its tracks. Except for 
>the  hammershank, which if the LO is close enough, has enough momentum to 

>go the remaining distance.

And if the hammer and shank are light enough.  I never thought about it before but here's another reason to use light hammers.

>  On its way back down from the string, it is checked not by the rep 
> spring pressure in drop, rather by sitting itself back down on the jack 
> which has not moved from its pre-LO position, because escapement never 
> occurred. The most obvious part of the consolation prize is the hiccup
we 
> all know and love, when the hammer bounces (albeit, slowly) off the jack 

> and grazes the string again.
>
>I've always believed that one of the least easy of factors determining
the 
>threshold sound (ie, minimal LO) to control is escapement friction. It's 
>partly a function of jack alignment, jack/rep spring strength, knuckle 
>shape and leather friction, hammershank ratio and SW. It's nice to 
>inventory these factors, but I have often been in the situation of unable 

>to alter escapement friction nearly as much as I'd like.

I agree.  The adjustments (or lack thereof) in the action haven't been set up to allow us to precisely control this escapement friction.  We can tweak it to some extent by rep lever and jack position and by knuckle lubrication.  But the results are difficult to control with precision and we haven't really developed good techniques for measuring them (at least
to my knowledge).

Also, we don't have a mechanism for altering this escapement friction with the strength of the blow.  Usually where it's an issue is for very soft playing.    Pianists would like to have the escapement friction be very low (but still detectable).  This is perhaps why a good upright action feels so good for very soft playing.  This escapement friction is low because there's not a lot of hammer weight on the jack.   But many pianists seem to like to have a very definite escapement feel for harder playing.  So, on a grand, if you try to achieve this reduced escapement friction for soft playing by tweaking things, like setting the jacks a little faster, or heavily lubricating the knuckles, etc. the pianist may not be happy at mp or mf levels of play (which is also why I think that pianists aren't as enthusiastic about an upright action at these levels of play).  What would be desirable I think is a variable friction escapement - low friction at soft levels of play and increasing friction with increasing dynamic levels.  Now, if I can just figure out how to design that .....


>Escapement friction is a turnstile. You can reduce the hiccuping by 
>fattening the LO. But the size of this friction determines the quietist 
>sound one can get out of a piano. So Del, would the lubricity of the jack 

>top make a big difference here?
>
>Bill Ballard RPT
>NH Chapter, P.T.G.


I also wonder about the friction of the leather on the rep lever against the drop screw.  At very soft levels of play I wonder if this is a significant factor?

Phil F