[pianotech] Bouncing Bostons

[Ron Nossaman]

William Monroe wrote:
> Yep,
>  
> height is good - slight gap at let-off (mm or 2) and angle is good, 
> (increasing resistance when pushed further into check, etc.)  And, 
> checking is just good all the way around.  When this hammer rebounds, 
> it's rebounding with unchecked force from the rest rail (so it 
> appears).  Rep spring strength can't create this kind of velocity.  
> Basically, if you closed your eyes you would suspect 
> let-off/drop/aftertouch based upon the double striking sound you hear, 
> but it's a full distance rebound.

My take is that the key stroke isn't bottomed out on the 
staccato blow, so there is no let off. The hammer rebounds off 
the string with the jack still under the knuckle. Since the 
back check hasn't "checked" all that rebound velocity, the 
hammer levers the wippen and key on down until the key end 
hits the back rail cloth. The hammer continues down past that 
point, compressing the knuckle, capstan cushion, back rail 
cloth, springing the key itself slightly, until it hits the 
shank cushion. That cushion is intended to catch the shank as 
the hammer drops out of check, and even then will bounce some 
as the knuckle, back rail cloth, etc, decompress. Add the 
considerably higher velocity of an unchecked hammer rebound, 
and you get a big bounce from the shank cushion as well as all 
the other pre-loads. If the shank cushion is firm enough, it 
can bounce the hammer back to string contact.

Did you ever see the hammer rest rail on the high end 
Disklavier verticals? Each shank has it's own little lightly 
spring loaded pneumatic piston to minimize bounce. In this 
case, it was to keep the sensor from recording an 
unintentional hammer stroke, but it's the same principal. On 
most, or at least many verticals, you can get that double 
strike staccato without trying too hard.

So the problem is the bounce. A free hammer center will 
aggravate the problem, but it's not the key balance rail hole, 
or the rep lever pinning, spring strength, or anything to do 
with the back check because they aren't involved. After I was 
satisfied the hammer centering was decent, I'd play with the 
blow distance next. Shank height relative to cushion at rest 
will determine how much "pre-load" is available for bounce, 
and ought to have some affect. The firmness of the back rail 
cloth will also have some affect. Then the cushion itself. Is 
it a tall loose felt, or compacted with a strip over the top?

I don't have a simple canned answer here, but I'd try to spend 
the time looking at logical possible causes instead of 
grasping at straws. I get to the straws often enough as it is 
without starting there.

A view from the cheap seats.

Ron N