[CAUT] fulcrum of a key

[Don Mannino]

Fred,

Another interesting topic, Fred.  One that I have investigated a little,
but certainly not exhaustively.

I believe it is safe to say that the fulcrum axis line is ALWAYS moving
during play.  As far as I know, it moves front to back depending on the
shape of the rail under the punching, and angle of the key in relation
to the rail top surface.  It can also move up and down (well, in an arc
actually) depending on the shape of the balance hole in the key
(cylindrical or conical). This last point is why a nice key balance hole
easing with a tapered tool from above really improves the tone and feel
of the action.

When designing the action, ratios are based on the pin location but in
reality, they do scootch around quite a bit.

Balance rails normally have a bevel on the front and back to allow
clearance of the keys during use, but the bevel is almost never directly
under the punching.  So it doesn't help much to add precision to the
balance axis line.  If a piano has that bevel cut lined up with the key
pins, it would have a similar effect as cutting the punchings.

The 1/2-round changes the way the key rolls, but I believe it actually
reduces the amount of fore-aft movement of the pivot line as compared to
a normal rail.  It is my feeling that it reduces the ratio change as
compared to a normal rail, and that may be why it was adopted in the
first place!  As for whether this is part of the "accelerated" action
concept, well, it's dangerous to mix marketing with engineering in that
way . . . . :-)

It seems that David Stanwood's nifty idea does force the rotational axis
to become more stationary, although it seems there are other drawbacks
to the system, such as increasing the vertical movement of the key on
the pin.  This is not a desirable motion to have, although it is
probably OK as a trade-off for improving the touch in other ways.  The
amount of up-down sliding must be pretty minimal, even at the most
extreme settings.

So, perhaps the central issue of your topic is: Is it desirable for the
rotational axis of the key to move fore and aft during play?  Now that's
a question.

As for the firmness of the balance punching, well, yes, harder is better
for the touch.  I think we'd all like the precision of metal washers
under there, but between the noise and the damage to the pianist's
tendons, I think we prefer to live with the vagueness of nice firm
bushing cloth.

Don Mannino


-----Original Message-----
From: caut-bounces at ptg.org [mailto:caut-bounces at ptg.org] On Behalf Of
Fred Sturm
Sent: Friday, October 02, 2009 12:32 PM
To: College & University Technicians
Subject: [CAUT] fulcrum of a key

	I've been thinking quite a bit lately about just exactly where
the fulcrum of a key is. One thing that got me thinking about the
question was David Stanwood's adjustable ratio design. It uses a fairly
sharp and precise fulcrum, that slides from behind the balance pin to in
front of the pin, a total of 8 mm, and changes the action ratio in the
range of from about 5:1 to 6:1. This is from about 2 mm behind the pin
to 2 mm in front.
	I have just been working on an action where I made use of sliced
punchings: felt balance punchings trimmed a bit (I bought them from
Pianoforte Supply, Crescendo punchings "accelerated"). They are sliced
about 1.5 to 2 mm from one side, so that there is a straight line, which
is put to the front of the key. That reduced touch weight a bit (by
changing the ratio), but not nearly enough, so I added half card
punchings glued to the bottom of the key (glued to the behind side of
the balance hole). Those two things put together got me a reduction of
about 10 grams. (It was an action that I would normally have switched
from 15.5 knuckles to 17.5, but all the parts were near new, so I
thought I'd try something else).
	Now I figure that my half cardboard punchings are, actually,
putting the fulcrum pretty darned near the center of the balance pin. So
it gets me wondering where, exactly, the effective fulcrum of a standard
set up is. Obviously it must be somewhere in front of the pin. Since
using felt punchings with about 1.5 mm trimmed from the front changes
the touchweight measurably, it seems like the effective point might
actually be near the front of the felt punching.
	As an aside, but somewhat relevant, the Steinway accelerated
half dowel bearings actually have a moving fulcrum (as the key bottom
rolls along the bearing), which "accelerates": it changes the ratio as
you depress the key.
	This question has some relevance to issues like what kind of
material is used for the balance punching (relatively spongy felt versus
crescendo, for instance), how high paper and cardboard punchings are
stacked, and maybe some other things as well. It may explain why
geometrical measurements of kay ratios and action parts don't predict
the actual action ratio (since we typically use the center of the
balance hole as the key fulcrum).
	Has anyone out there given this much thought and come up with
some conclusions?
Regards,
Fred Sturm
University of New Mexico
fssturm at unm.edu