Keyweights and moment of inertia (Long clarification)

[Rob Kiddell]

> Rob,
> 
> In regard to why your keyboard felt easier to play after you moved
> the weights forward, I have another speculation.  When the weights
> are moved forward, they are also reduced.  This reduces the total
> key weight supported at the balance rail.  If the bushing and pin
> alignment is such that a heavier key causes more binding at the 
> pin, then perhaps that could cause a heavier feel.  If that were
> the case, it would also increase the difference between down-weight
> and up-weight, so it should be easy to check.
> 
> Robert Scott
> Ann Arbor, Michigan
> 

Robert, 

	The key weighting was the same as it was with the larger 
weights closer to the balance rail; in effect, I re-weighed the 
keyboard to give the same up and down-weight measurements, but 
accomplished this with less lead overall in the keys. The moment 
of inertia problem for the key seems to be cancelled out by the 
much greater hammer inertia ratio, and despite the R² (or 
thereabouts) increase in inertia based on distance from the balance 
pin, the action didn't have that *grunt* you needed to play it 
originally when it had more lead in the keys. 
	Friction, either at the key bushings, or in the wip and knuckle 
geometry were acceptable, and again, the touchweight both before 
and after the re-weighing was within spec. 
	The real problem seemed that there was excessive lead in the 
keys to balance the action, 4-5 ½" leads per key in the bass, 3-4 
in the middle, and 0-2 in the top sections. Replacing hammers was 
not the solution (they were voiced very well, changing them would 
have been a crime!). I had analyzed the action geometry from a 
number of angles, and aside from some correction in the 
regulation, all the parts seemed as aligned as they could be, 
barring parts replacement. 
	This was (and still is) a new instrument, and the customer was 
complaining of "heaviness" in the action, and touchweight was not 
the culprit. Nor were the dampers, as I went through this with her in 
her home (playing with and without the dampers). Another problem 
I noticed was that the keys made a lot of "thump" when released, 
particularly in the bass. When you removed the stack and lifted a 
bass key from the keyframe, the bloody key weighed a ton! I've 
corrected excessive leading in old grands by changing hammers, 
moving capstan lines, and subsequently re-leading keys removing 
several leads per key in the process. In this case, I couldn't do 
options A (hammer replacement) or B (capstan movement) 
because in my opinion, this wasn't the root of the problem. It was 
the fact that despite balanced up and down weights, the keys had 
lots of lead in (and this is were I am speculating...) the wrong 
place. Re-weighing with 3/8" leads farther forward allowed me to 
remove one and sometimes two ½" leads near the balance rail. 
	Now, don't think I just did all of this without experimenting on 
one or two keys first! Or without changing hammers on one or two 
notes to see what difference it made!  8-)
	Hammer replacement proved difficult, as all replacement 
hammers I had were heavier than the ones on the action, which are 
light hammers to start with. Heavier replacements mean more key 
lead... n'est çe pas?
	I popped out a ½" lead near the balance rail and experimented 
with lighter leads farther along the key. One 3/8" lead between the 
front rail slot and the key front cover (drilled into the underside of 
the key) proved the most effective. Felt better inertially (there I go 
using that darned I-word again!) Key didn't thump on release as 
much as its (non-re-leaded) neighbors. 
	So, I  proverbially "went for it"... re-leading according to this 
method. Removed a coffee-can full of ½" leads, and inserted a 
packet of 3/8" leads. Even the action was easier to lift in and out of 
my car! 8-)
	In the end, the client was happy (as far as I know from my last 
phone call), I seemed to find a difference (or maybe I just wanted to 
feel like I had accomplished something), and the action was less 
noisy.
	Now, what's all this about R²?

Regards, 


Rob Kiddell, RPT
atonal@telusplanet.net

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