My thanks for the replies concerning my question about "strike ratios." I do
know the difference between static weight and inertial resistance. My
question concerning
>.For those of you not familiar with the term strike ratio, it is the
>amount of weight needed to balance one gram of hammer weight.
actually meant merely "are you describing a method?" and was
cleared up by David S. (he was describing a result, not a method). I
subsequently re-located his printed materials on the subject, and refreshed
my memory of his method. I have no criticism of it; it seems to me like a
perfectly sound and practical way of dealing with friction, weight, and
inertia. I still think there are a couple of other approaches that have use.
It sounds like a lot of you have had experiences similar to mine - that
pianists, given reasonable friction, are MUCH more sensitive to inertia than
to weight (not ignoring evenness and speed of repetition, of course). I have
had pianists like a 70-gram (!) low inertia keyboard better than a 50-gram
high-inertia one, and I have had pianists who are used to playing on
medium-inertia Steinways dislike a 50-gram low-inertia Boesendorfer. because
it felt "squirrelly."
>the germinal problem is not one of unexpected
>keydips and blows, but rather that action resistence gets blown around the
>bend anytime we change parts (hammers, shanks, knuckles, reps, you name it).
>This is where the ability to read action leverage by mass is so important.
[Bill Ballard]
We certainly need both leverage and mass when gauging inertia, but the
leverage doesn't have to be read via the mass, and the mass doesn't have to
be read at both ends to be useful. I can think of three methods of measuring
leverage. Each has advantages and disadvantages. 1)David Stanwood's method of
weight/weight, which seems practical for the actual rebalancing work. 2)There
is also the possibility of measuring the ratios of each leg of the leverage
system and multiplying them together. This doesn't take too long either, and
has the advantage of getting acquainted with each link in the chain. 3)
Measuring the hammer movement versus the keystroke (not including
aftertouch). This is quick too, and the dip part can be done to any level of
accuracy. For instance, it can be done with a ruler or a depth gauge or a
dial indicator.
All three methods can be done on more than one note and the results averaged,
or on several notes if the leverage changes from treble to bass. Each method
will come out with slightly different results. The leverage actually changes
as parts move, and we're dealing with arcs rather than straight lines.
However, the relative is more important than the absolute, and each system
has its place. As in all areas of piano work, an understanding of the
concepts is what leads to methods effective for each of us. As in regulating,
it's not by the numbers - when we have this many variables to play off each
other, there's still a bit of an art, and experience helps us decide which
variable to favor. For instance, the hammer mass is important not only to the
feel of the action, but its relationship to the _string_ mass and period
of vibration has a lot to do with the tone. If we put on heavier hammers for
a particular tonal quality, do we want to change the leverage to fix the
resulting greater inertia? where? Or do we have wippen support springs which
can reduce the weight on the key without adding inertia? A little of both?
Do we want to use the information to re-establish the original design, or
modify it to suit a particular client's ( or our own ) particular needs or
desires?
For the mass, one can measure the hammer head alone, or the hammer head and
shank assembly pivoted at the flange pin (a la Stanwood). The head weight is
the most important because it is so far away from the pivot. David's method
is more accurate on an absolute basis, but it's still instructive to measure
heads if they're off the shanks -- I think it's good to know a heavy set from
a light one in various stages of preparation, especially when you're about to
hang a new set. Just as David has empirically derived values for light,
medium and heavy strike weights, others of us know light, medium, and heavy
hammers.
If we know the leverage and the hammer mass, the mass of the front of the key
is implicit in a wippen-spring-less system (I know, there's the wippen and
shank, but we can almost ignore them as constants), and we have enough
information to judge the relative inertia. For a quick assessment either of a
problem action or of one due an overhaul, I weigh at least A0, C4, and C8
(pivoted on their shanks), and dip vs blow. It takes just a few minutes and
gives me a pretty good idea of where I'm headed. If the friction is high, I
also check one set of parts for proper pinning, and if it is okay but the
friction is still high, I check for rubbing friction due to bad geometry. I
suppose if you use the same method every time, you could multiply the
leverage ratio by the hammer mass, and come up with a set of inertia numbers
(graduated from bass to treble) you could compare from action to action.
Weighty matters.
Bob Davis
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