[CAUT] S & S D rest cushions

[Fred Sturm]

On Aug 16, 2009, at 8:40 AM, David Love wrote:

> I have to disagree with those who think the amount of space below  
> the hammer
> shank is unimportant or not likely to affect performance.  While the  
> rest
> cushion spacing may not come into play on most types of repetition
> requirements, a set up that allows the shank to over rotate below  
> the set
> position will increase the amount of time that the jack requires to  
> reset
> under the knuckle.  While it may not be an issue for most performance
> requirements, there will inevitably be some types repetition where  
> excess
> air space below the cushion will reduce repetition speed and  
> efficiency.
> Moreover, with the hammer being able to rotate more downward (and  
> depending
> on how large the gap is) there is the chance that the backcheck can  
> catch
> the hammer shoulder in certain instances (especially in the bass).   
> I would
> argue for reducing that gap should you find yourself in that  
> position by
> building up the rest cushion with an appropriately dimensioned piece  
> of
> felt.
>
> David Love


	Although what I have been writing in this thread has been aimed at a  
specific and fairly rare problem, I agree with David that minimizing  
space between cushion and shank will improve repetition in general  
(though maybe not terribly noticeably). It is very much like the felt  
limiting the jack's backward travel, as in the felt in the rep window  
or various designs for uprights (the Yamaha rail with felt limiting  
jack travel). High speed videography shows that if the jack can move  
farther, it _will_ move farther, and thus it will bounce more (will  
hit the knuckle and rebound, often several times), and will be less  
likely to get back under at the moment you want it to. Similarly with  
a hammer on the end of a shank, if it can move that extra distance, it  
will be wasted distance that has to be made up somehow. Exactly what  
will happen I'd hesitate to predict - haven't seen high speed of that  
particular thing - but it seems certain that it would slow down  
repetition in some cases or make it less predictable.
	We know with certainty that shanks strike the cushion felt on normal,  
not too hard play: think of those rails with the nuts that hold them  
in place. The nuts get loose, and the customer hears all sorts of  
clicks and thinks the new piano has degraded into a heap of garbage.  
Those clicks are caused by the shanks hitting the rail with enough  
force to set it in motion. And it happens even if the rail is rather  
far from the shanks.
	There are a lot of things that happen that we don't think could  
possibly happen, with action parts moving beyond what we expect and  
shivering and whatnot. An interesting example is the top of a jack  
hitting the bottom of a hammer flange. I didn't believe it was  
possible until a pianist pointed out the noise, and I looked and saw  
the mark on the bottom of the flange. It took some heavy, sharp  
playing to do that, but it happened (cure: crescendo front punchings).
	
Regards,
Fred Sturm
University of New Mexico
fssturm at unm.edu