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Barbara
This, from the archives, might be what you're looking for.
Tom Cole
https://www.moypiano.com/ptg/pianotech.php/1995-August/002642.html
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Tight centers in Young Chang (and other) grands
Don_Mannino@yca.ccmail.compuserve.com
Don_Mannino@yca.ccmail.compuserve.com
<mailto:Don_Mannino%40yca.ccmail.compuserve.com>
Mon, 14 Aug 1995 15:09:50 -0400 (EDT)
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Vince Mrykalo wrote:
>>Don,
I know in the past there was a problem with hammer centers freezing up
in YCh grands. In my experience, an application of protek was only a
temporary fix. Has anybody figured out why this was happening? Do
you have any information that would shed some light on the problem? I
have found also that using the broaches and repinning, if the
repinning was a little snug, working the part would loosen it up
satisfactorily. But then I wonder if it would stay that way or again
tighten up later. If you would like to respond to the group on this,
go ahead. Thanks!<<
Don Mannino's reply:
I went to Korea in September 94 to finally work through this problem
and help get it solved. Our action department had been making minor
adjustments to procedures, and what was needed was a more major
change.
The bushings themselves were unstable. Although the bushings were
sized correctly, the wool fibers were being compressed into position
by wet sizing only, and the fibers didn't stay put under heavy use.
The combination of heat and shocking of the felt caused the fibers to
come back out of their squashed positions and tighten up against the
pin again.
The reaming / burnishing process disturbs the fibers sufficiently to
leave them stable. I have heard of only one occurrence of our parts
tightening up again after reaming, and the circumstances in that one
case were not ideal (inexperienced technician, possible contamination
of the cloth, etc).
YC has a test chamber where the action can be pounded for a
predetermined number of keystrokes while the temperature and humidity
in the chamber are cycled up and down. During our work we ran actions
for 200,000 keystrokes (about 2 days constant playing in the machine),
then when the final procedure was settled on they ran the test for
2,000,000 keystrokes (after I came home - this takes about 2 weeks).
We tested different cloths, glues, pins and fitting procedures, and
found that the _fitting procedures_ had the most impact on the
stability of the action center fit. All the cloths and glues
exhibited the same symptoms of tightening under use, to varying
degrees, when fit only by shrink fitting, or wet sizing.
Lubrication of the parts helped this, but didn't solve the problem.
We tested a variety of lubricants, everything I know of except
Steinway's Emralon dip.
We tried to map the movement of the fibers by noting their locations
before and after under a microscope. Although super precise movement
mapping of every visible fiber wasn't practical, it was clear that the
fibers in wet-sized bushings would move around and change position
after being played. In the more stable, burnished bushings the fibers
hardly moved at all.
Another interesting point - once the fibers move around and start to
bind on the pin, the wool wears very clear grooves into the pin. The
wool fibers leave what look like impressions around the circumference.
The only procedure that was 100% reliable in the factory was reaming
and/or burnishing to achieve the final fit. While centers that were
wet-fit only, using the best combination of shrinking solution
lubricant and procedure, tightened up an average of 2.5 grams (range:
.5 - 5 grams) after 200,000 keystrokes, the reamed flanges _loosened_
an average of .5 grams. Unfortunately the problem with reaming is
that the flange can become spongy and soft, so wet fitting combined
with burnishing was settled on as the best procedure.
The more rough the surface of the reaming tool, the more the center
will loosen up with use. The best broaches have only slight roughness,
and the burnishing / smoothing step after reaming is important for
keeping the fit stable.
Another variable that contributes to loosening is the quality of the
cloth - cheap cloth tends to loosen up more after reaming than good,
close weave, high density cloth.
If you work a well-reamed part _really_ hard you can usually get it to
loosen a little - maybe .5 to 1 gram looser than right after pinning.
Part of this is from the wool fibers being bent into unhappy positions
when the new pin was inserted, and part of it is the inevitable
fluffiness of the surface fibers contacting the pin, which become
packed down during use. The only solution is to test for loosening
when you establish your procedure on the first few parts in the set,
and adjusting the procedure to minimize the loosening.
Try changing to the broach of the same size as the pin you are going
to use, or even the broach that is .001 larger than the pin, with only
1 pass of the rough part of the broach and a few seconds of vigorous
burnishing. This will improve the stability of the fit. Another thing
you can try is to wet size the cloth first with a pin .002" smaller
than the final pin, then do the broach work.
Don_Mannino@yca.ccmail.compuserve.com <mailto:Don_Mannino@yca.ccmail.compuserve.com>
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