Hi Ric
I've been gone this week. First, transporting the little redesigned Stwy
O to Alameda for the class, then teaching the class & visiting with friends,
returning piano etc,etc add exhaustion. It's Friday & I'm glad so I havent'
been ignoring this
Hi Dale
This argument about ribs not supporting crown has bothered me from the
first moment I heard it. And after reading Nossamans well written
article in the latest Journal I think I know why. Ok, nobody questions
that in a CC board ribs do not provide beam support for the load. But
that's not quite the same thing as saying they don't support load in a
different fashion.
Hey they add something . If that wasn't true every C. C. Board wudda
caved on inception.
The same thing goes for the crown argument. And
that's where Ron's article comes in.
I'm not sure I know which article this is. I'll find it. what month?
About 2 years ago I posted a couple threads with some drawings trying to
explain why I thought the ribs in a CC board had similarities to a
cable in the sense that they attempt to constrain the board from
expanding... so the panel has to bend instead. That very resistance to
the panels expansion is every bit as much a load support but in an
entirely different way. What the kicker back then was, was that I tried
to argue that the ribs strain (note the word usage) against the
expansion forces from the panel. I was told then that no.. the ribs
don't strain.... they simply bend against their better nature.
As I read the other posts on " Strain" & I have to see the ribs being
pulled up from there flat orinetation by panel expansion as ......strain? If
some one bends me out of shape I feel ....strained
Enter
Ron's article disclaiming the buttress arch. In that article he shows by
experiment that the top half of the ribs not only bend, but they expand
lengthwise. That expansion is critical to his whole argumentation
(which by the way made perfect sense to me).
But that same rib expansion shows conclusively that the ribs do strain,
and significantly so against the expanding panel.
The strain is being initiated by the panel expansion
If you stop to think
about it this only makes sense. If the ribs can not strain lengthwise
at all, then neither could the panel crown, yet if they strained equally
through their height then they would not constrain the panel at all. It
is because they DO strain ... more on top and increasingly less towards
the bottom combined with the panels compression that crown and crown
strength occur. And it doesn't really seem to me to be so much a stretch
of the mind to imagine mathematical explanations for all this that
would fit very nicely into design thinking. The height and width of
ribs don't add up to combine in a kind of beam strength / mass
relationship... but rather a kind of strain strength / mass one.
I think I follow you. Frankly it helps me to think the whole assembly
as plywood
One thing is clear about load support in CC boards. The more you push on
it, the more it resists... until its overloaded of course. But until
that point there is definitely load support and the ribs are definitely
part of that... just not in the sense of beams.
This is true but the ribs are adding stiffness & mass & mass is a often
neglected piece of the discussion at times. Ie. I've heard many C.C. board
with little crown or bearing that really sounded to my ear,very good, enjoyed
them greatly. So what's making them work? rib stiffness/ panel assembly
stiffness & being glued as an assembly to the rim adds stiffness.
When I load up a RC & partially panel supported board, there can be a
very powerful strong non-linear resistence during the pre stress bearing set
up. As I pound the board down, slipping a wooden wedge between the central
plate strut, the resistence to further deflection stops at about 3 to 4 mm even
if I continue to pound really hard with fist on bridge. It feels like I'm
pounding on a gym floor at that point. The stiffness in that develops in a
Sitka panel at 5.5 to 6.0 E.M.C. can exert a lot of stiffness factor which
really quite amazing. & So is a purelay C.C. panel at 4% as well. I got to
thinkling about how strong spruce expansion is when considering ,that a
thoroughly dried C.C. a panel can bend 1 2 or more ribs for really quite along time
and sound quite good. This is amzing.
The longevity issues & reliability issues of C.C. boards are always the
big question mark. Ie the G-2 Yamaha I wrote about thinning the panel on is
a C.C. board but with sitka spruce ribs. Yikes How long can that board
convince those stiff suckers from sucking the life out of the crown, & it's only
29 years old. In 20 years it may be ready for the dumper, as are many mass
produced pianos.
In my boards I know panel expansion gives some non linear strength
but I love the way they sound. I've got a pretty stiff rib set under there
as well. I'm wondering if the non linear panel qualities are noticed or
utilized more predominately in the large bottom end of the panel. I've been
considering that the shorter ribs in the treble end, Say the last 5 ribs, are
getting short enough to perhaps add beam strength as they don't crown much Even
under panel compression a flat rib doesn't normally show much residual crown
when strung up & the noticeably shorter spans the board makes from belly rail
to rim also adds a bit of stiffness but it seems to me that it is the whole
of the structure itself which is seen as a mystery as to how to design one
from an engeneering standpoint & understanding it. Hey it works, we can't argue
with that...but how well, how long....this we can debate till were blue in
de face.
Dale
Cheers
RicB
------------------
Dale,
Those ribs were originally built into a CC board. How can a
compression crowned board get mechanical support from the rib
scale, however "good" the scale looks? The ribs in CC boards
resist the crown that panel compression is trying to form and
maintain, and just put more compression load on the panel. -----Ron
No, I get all that Ron, but if the ribs are built significantly taller &
of stiffer material by design then more panel compression can be taken
out of
the equation. Another thought is, & I've witnessed this quite a few times
is, that Stwy A's (1 & 2"s) in general can produce a pretty wonderful
sound
even with a flat or flattish board providing there is some small but
consistent bearing load still intact so in this case it would seem that
there are
enough impedance factors about the rib scale to make the system work
rather well.
Ok maybe a freak of nature but it happens fairly frequently.
About 5 years back I had such an long A I was going to resell it. It
had
Steinway hammers which were quite soft & made it sound really good.
It was
hard to imagine that a new board would make it sound much better. It truly
sounded glorious but it was a spec job & I don't sell old boards very
often. I
didn't do any thing to the action until later for a really good A b
comparison.
So I built a board with the same number of ribs making them crowned at
about 60 ft. Made em taller but not much. Used sugar pine in the bottom &
yellow pine in the top. The sound was cleaner and the sustain was
about the same
which was awesome. It just had it!!
I attribute much of this to the original basic rib scale design.
Something
was working or several things were. Do you see what I'm saying?
I'll crunch some numbers & see what I got. I greatly Appreciate the
design sharing & information swap.
Thanks
Dale
It's an entirely different system. Do a bearing load analysis
on the ribs as load carrying beams and see what the numbers
say. They'll say that the rib scale isn't adequate to support
bearing without substantial panel compression support.
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