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Personally, I think Terry is right when he makes the point that rib =
crowning and compression crowning are just two extremes on a continuum =
of possibilities. Although it may indeed be a trivial distinction, the =
feathering at the ends of a rib on a compression crowned board, where =
such exists, means, functionally at least, an element of similarity to =
a crowned rib. =20
Appears to me you are pointing out a similarity rather than a =
distinction. Yes, both designs are commonly made to be more flexible =
around its perimeter.
As far as I can see, the only truly compression crowned boards would be =
those with unfeathered ribs, which I have seen on just a few pianos, =
one of which was a small Mehlin, if memory serves. =20
One could easily build a "truly compression crowned board" with either =
tapered or untapered ribs. One could easily build a rib crowned board =
with both types of ribs. Rib tapering does not determine soundboard =
construction.
If one's measure of rib crowning is a rib which has a variable =
thickness along the unfeathered area and is taller than wide, =20
That is not a measure of rib crowning. Rib crowning is a product of =
shaping the panel side of a rib into an arc. Rib taper does not =
necessarily have nothing to do it, nor does rib cross section.
then there are any number of older pianos out there with this =
characteristic. Particularly noteworthy, are the pianos of Chickering =
from about 1900 where, in their production they explored the use of =
various radii, altered rib scaling, shorter or longer ribs, larger or =
small average cross sections, larger or smaller treble sections, etc. =
The late 123 has one rib, again if my memory is correct, which varies in =
thickness by almost a quarter of an inch. Much of what is represented =
here as novel, again with all due respect to those who think so, appears =
to have been anticipated a century or so earlier through the efforts of =
this firm and the present "new" school of design now clamorously present =
as "modern" seems, in amazingly similar ways, to recapitulate the =
technical evolution of this firm. =20
I have read repeatedly on this list how pretty much all of what is being =
done with rib crowned soundboard designs and rim modifications, etc. =
have all been done before. These have been discussed ad-naseum. =
Certainly some new thining is also going on here, but the nuts and bolts =
of it are clearly recognised as having been used again and again in the =
past. I think what might be novel here, at least considering the last 80 =
years or so, is that anything different from what that famous NY =
manufacturer does could possibly be worthy of consideration.
This may, or, may not, be fortuitous but it is certainly useful as a =
counterpoise to the "standard" approach typified by Steinway. =20
The Boston school, as it were, of piano manufacturing, versus, say, =
that of New York, as a whole displays a greater similarity in this =
regard to English pianos while that of New York appears somewhat more =
similar to those of German ones, although, of course, Mason & Hamlin, in =
some aspects, at least in their independent production, is an exception =
to this generalization. Broadwood's ribbing, at least on the four or =
five mid 19th century pianos I have observed appears very similar to =
that of Chickering, even though I did not measure the rib heights =
looking for variations there. Nevertheless, I feel fairly comfortable =
asserting that rib crowning has a long history, predating even 1900 and =
would be more associated with English pianos in this regard; German =
pianos may well have favored the thinner, wide rib used in compression =
crowning. =20
It may well be that, in the United States, the collapse of the =
piano industry in the third and fourth decade of the last century and =
the subsequent unchallenged preeminence of the New York school has led =
to the illustion that there has been no alternative to the crowning =
methods used there, which I think is markedly incorrect. Rib crowning =
has a long history and appears to have been well explored.=20
I would agree with this, however, I might suggest that whereas "rib =
crowning has a long history and appears to have been well explored", it =
has perhaps not been thoroughly explored in the past.
=20
There is no doubt that Chickering pianos exhibit substantially =
less cracking than those of Steinway. This appears to be consistent =
with the claims of the RC advocates. It is interesting to note that =
cracks in Steinways, in general, are significantly wider at the top =
surface of the board, as most know, than they are at the bottom which =
seems in keeping with the compression crowning view of tension on the =
top of the board and compression on the bottom; cracks found on =
Chickerings show dramatically less widening through the section of the =
board. =20
Although I am not sure they qualify as RC&S boards, perhaps or =
perhaps not, these boards certainly appear to be RC. =20
I have urged the view before that it is inappropriate to apply =
simple elastic moduli indiscriminately as is done here when using them =
to make a few modest calculations which are then generalized to the =
conclusion that damaging levels of compression are inevitable, much =
less, even achievable in ordinary soundboards, be they RC or CC. First =
and foremost as the intrinsic condition of every soundboard is one of a =
very complicated, idiosyncratic, triaxial stress distribution and not =
one of uniaxial or plane stress, plane stress moduli are inappropriate =
to the facts, yet these very moduli are used by those urging the =
validity of compression damage. =20
What is the Fiber stress at proportional limit which is what is =
quoted for disabling cross grain compression? It is simply the point at =
which elastic action ceases and the material will be incapable of =
achieving, when freed of compression across the grain, its previous =
dimensions. This by no means is a point of "cellular crushing" or =
destruction which is so often claimed here. The net result of such an =
effect, should it occur, as Richard Breckne has just pointed out in a =
recent post is, simply an increasing density and increasing strength =
which, no doubt, increases the acoustic velocity somewhat, and, again, =
may allow an actual improvement in the perceptual aspect of tone quality =
or have other beneficial effects absent other structural failures in the =
soundboard panel. =20
As I urged last year those interested in such things should take =
the various manuals which, at one time, were suggested to contain the =
material upon which such a claim of damage could be based and =
familiarize themselves with them. One will find, should one do so, for =
example, that the coefficient of variation were this simple property, =
that of cross grain compression strenth, applicable, according to The =
Encyclopedia of Wood is 28% which means that attempting to make any kind =
of precise extrapolation from the results of such a calculation is =
unreliable and inappropriate. As for an empircal indicator, I have =
never seen, in thousands of pianos a significant change in the =
circularity of nosebolt holes found here and there in soundboards, =
which, were such a level of damaging "compression set" as is frequently =
assumed here, underway, would be likely to develop a significant =
eccentricity and become elliptical with a major axis parallel to the =
grain. =20
In my opinion, crack formation in older, high quality soundboards =
is, in most cases, the result of shear failure and stress concentration, =
and not an indication of previous compression ridging, followed by =
drying and the opening of a crack. Compression failure, per se, in my =
opinion, is more likely to be found in more modern boards that have been =
ribbed at too low a moisture content.
Loading of a crowned, or even uncrowned, soundboard along both =
sides the bridges asymmetrically by the downbearing pressure, and their =
interactions, inevitably introduces additional significant shear on =
both sides of the bridges, in every case, which ranges with moisture =
fluctuations and downbearing load. Stress relief over time results in =
the very commonly found long crack or two or three or four, etc. =
running sometimes as a chord between the ends of the bridge or, =
sometimes, parallel to it a few inches away. Where the shear is =
greater, for example boards with a tenor bridge, a characteristic field =
of cracks may well develop. According to the selfsame Encyclopedia =
indicated above shear parallel to the grain is virtually the weakest =
property of wood. Combine the high shear levels in this area of the =
board with the weakness of the material in this regard, high moisture =
levels, the possiblity of detached ribs and throw in an additional =
factor: the stress concentrating effect of nosebolt holes that are found =
commonly in this area and you have an efficient mechanism for crack =
formation. It is this shear field that accounts for the non-random =
placement of cracks in boards and their association with nosebolt holes =
as it is extremely common to find a crack originating in a nosebolt hole =
and, sometimes, even the screw holes used for screws attaching the guide =
rail . These are not random events. =20
A second mechanism for crack formation exists in the classic =
interaction of the differences of radial and tangential shrinkage and =
expansion along with the effects of grain angle which produces warping =
in free boards. Even though the flitches are laid up with grain angles =
aligned to minimize this, these efforts are only partially effective and =
differential stresses, unique to each individual flitch are still =
present in the completed panel which work their effects over time. =
These effects are to separate the individual flitches along the joint. =
I would argue that most cracks fall into either of these two categories =
and are not, themselves, the results of "cellular crushing or =
compression set", although a lot of newish pianos nowadays very plainly =
show compression ridges. =20
I don't mean to imply that soundboards cannot undergo significant =
functional failures; of course, they can and do, but these are not =
failures of the wood material itself but, rather, structural failures of =
aspects of the panel assembly: the most important of which, in my =
opinion, is detachment of the soundboard from the ribs, bridges or rim. =
Build a few panel models and expose them to humidity extremes. Measure =
changes in crown. Observe crown/shape recovery. Such an activity is a =
good way to learn about "compression set". Soundboards can get squished =
with enough compression and enough time and do not recover to their =
origninal shape.
Regards, Robin Hufford
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