---------------------- multipart/alternative attachment
Hi John
Great post. I didn't get a chance to comment last friday when it came
in as I was on my way out of town. I've interspersed some comments below.
John Hartman wrote
My point is that once the soundboard is assembled the
ribs attached to a Panel Crowned (PC) board contribute to the stiffness
of the structure. If this were not true then a PC board would not have
any more resistance to deflection that a panel without ribs. I have
handled many soundboards some of which were built with the PC method.
When the ribs are attached to the whole structure noticeably stiffer. So
were does this stiffness come from if not from the ribs?
>>>> I see this as one of several stiffness factors, whether PC or Rc , that
contributes to an overall symbiotic effect totaling a soundboard structures
overall stiffness quotient
>> Furthermore, the idea that the small additional compression of the
panel
>> due to bearing will stress the panel of a PC soundboard more than
a RC
>> board is just not right.
>
>
>Ron wrote -compression? You mean in addition to the compression
> necessary to force crown in the ribs before the string bearing is
even
> applied. How then, are the ribs supporting most of the bearing load
in a
> CC board when the panel is already supporting the ribs? That's the
> question I'm addressing.
John
I am just going on the evidence I see. When you apply down bearing the
board moves down about 3mm. You can compute how much more (in addition
to compression of crowning) compression the panel is holding by figuring
how much this straightening of the crown compresses the panel. If you do
this you will find that bearing does not compress the panel to any
significant degree. This is true for PC and RC boards alike. At any time we
can find the compression levels of a soundboard panel by looking at how
much change in dimension (cross grain) there is when we apply bearing.
>>>>>I agree. Depending on how stiff the soundboard structure is I find that
typically the board flattens appox 2.5 to 3.5 mm measured at the central strut
approx., 2ish mm at the first capo strut, bout 1mm or less a the top & about
1.5 At the bottom tenor. During the pre-stressing for bearing(wedging the
board down at the struts) setup I find the board gets very stiff & resistent to
further deflectionat this point. This is another indicator of overall soundboard
stiffness due to each facet of the component stiffening effect meaning rib
to board interface, bridge crown, panel stiffness, rim effect etc.
As you say It doesn't seem the panel is going to compress significantly
with these minor increases from downbearing. I typically dry Sitka panels
before ribbing between 5.5% to 6% EMC out here in Calif. Many go to the bay area
with higher average moisture levels than the central valley where I am. I
have followed many of these pianos over timed see no sign of compression ridges,
cracks bulging etc. With other kinds of spruce with less strength across grain
, or containing too much soft spring/early wood, it could be a problem.
On that 48" rib you mentioned, the cross grain
> dimension of a panel that is 48" at 4.5%MC, will be approximately
48.5"
> at 12%MC. It takes half an inch of compression in a 48" panel
just to
> form the crown in a flat rib. That's a bit over 1% of the total
width.
> Immediate and permanent compression set occurs at anything over 1%,
> according to the literature.
>>> So then it looks like a safe level of E.M.C for panel crowning could
range from 5.5 % to 6%emc
While I believe that any board made this way is at risk to long term
deterioration the PC method as used at the current Steinway factory does
not compress the panel to the point of damage (above the elastic
limit). From my own experience with examining boards I have pulled
from relatively new (under 20 years) Steinways you do see most of the
crown intact
(Steinway does not use a lot of crown).
John I'm Curious . Why were these boards taken out?
My crude attempts to find how
much compression is involved indicates that about 1/3 the elastic limit
is reached when a PC board is crowned and reaches 6.5% moisture content.
This is
about 200 psi to create the crown. Yes this is a concern and I would not
build a board this way but the panel is not at the point were
were it can't hold the crown with some leeway for increased moisture (it
doesn't begin to get damaged until its above 11%EMC and even then in can
still support the load). As you know cross
grain spruce can withstand about 580 psi before it starts to fail. The
1/3 figure I found correspond well to what we know about the moisture
levels used to rib a board with the PC method in comparison to the
moisture level used with the RC method. It is a difference of 2 percent
which if viewed as the stress in a constrained soundboard panel comes
out to be a little less than 200psi.
>>.. This is interesting and good to know
If you have experiments or data to contrast with this I would like to
see them.
What does bearing add? Typically more than
> what it takes for ribs of that dimension (no panel) to be deflected
flat
> if they had been machine crowned. So the bearing adds at least as
much
> load as it took to bend the ribs.
No I don't think bearing ads mush compression to the panel for the
reasons I went into above.
Ron
Little stress on the panel, and the
> ribs carrying the majority of the bearing load? I think not. And
when
> you force that panel flat, so the rib is again straight, the rib is
then
> under no stress at all except at the top edge. What is it that is
> pushing back on the force it takes to push it flat? It's the panel
> compression. So please explain how the ribs are supporting most
of the
> bearing, with the panel supporting very little.
John
The best way I can find to look at this is to examine the evidence. We
have a PC board with ribs that are shallower than what is necessary to
hold the bearing. We put the strings on and they push the board down. We
find that the soundboard has not collapsed and holds the bearing load
more than we predicted looking at the ribs. We look at the panel and
find that it has not been significantly compressed from the bearing. We
recheck the deflection of the ribs and find that they are not holding
all of the load ether. Jumping to conclusions we think the panel must be
holding it - this is sloppy thinking and flies in the face of the evidence.
I think that the model that a few have been using to explain how the
soundboard works is, well wrong. We have been fed this idea that the
ribs are like floor joists and the panel is like the floor boards. The
ribs hold all the weight and the panel does none of the work. May I
propose an alternate model? The ribs and the panel and the bridge form a
single crowned unit that has pretty much the same stiffness along
the ribs as it does along the grain of the panel. It is like a crowned
diaphragm. When the soundboard is attached to the rim all of the
components form one structure well suited to support bearing. If you
want to have a complete picture of how bearing is supported you must
consider the contribution of the rim. As the diaphragm is pushed down
the rim holds it in and provides the added stiffness to resist bearing.
This theory at least makes some attempt to show how crown can improve
soundboard performance.
>>>>Not only that but it does help to understand where the overall stiffness
in the soundboard models we've inherited comes from. The soundboard is
somewhat of a laminated structure afterall. I posted on the increased stiffness that
the board takes on after it's glued to the rim last year but the idea was
minimized, However any one who goes to the trouble of building more than a few
boards quickly realizes the dynamic Increase in a soundboards stiffness once
glued to the rim compared to what it was out of the piano. It obviously becomes a
homogenous unit at this point. What I'm not saying is the success of the whole
system is dependent on this one thing. It's part of a whole
John
With the floor joist model all that can be said
for crown is that it compresses the panel. I say So What! compressing
the panel can do nothing to improve tone.
>>>>Hmmm. Maybe not the panel alone but the whole assembly. I always thought
of this as something like tightening a drum head & that by doing so a light
weight object becomes more efficient at producing more sound thru increased air
movement. Or basically what we all refer to as an impedance increase.
The soundboard we inherited has evolved to have equal stiffness both
along the ribs and along the grain of the panel. This makes sense
because it assures that the vibration modes of the soundboard will be
fairly circular in shape to best utilize the soundboard area. If you
start to design the ribs to be more stiff than the panel the vibrating
areas of the soundboard will begin to elongate in the direction of the
ribs. At some point if you continue to design the ribs to support the
bearing load, all on there own, you will make a soundboard less capable
of tone projection with small area vibrating out of phase. This would
defeat the whole purpose of supporting bearing in the first place.
>>>>>
One things for sure. I have enjoyed the discussion of older & redesigned
sound boards, wood strengths and various aspects of the craft on the list. One
thing I want to say is I know rebuilders around the country that are using
crowned ribs, with or without significant increases in rib height dimensions and
are somewhat modified copies of original of soundboards that sound very,very
good.
If anyone has had the idea that only a highly modified board is an
exceptable and legitimate attempt at quality piano rebuilding just isn't so. The
proof is in the sound. Build a few boards and draw your own conclusions.
DAle Erwin
John Hartman RPT
John Hartman Pianos
[link redacted at request of site owner - Jul 25, 2015]
Rebuilding Steinway and Mason & Hamlin
Grand Pianos Since 1979
Piano Technicians Journal
Journal Illustrator/Contributing Editor
[link redacted at request of site owner - Jul 25, 2015]
---------------------- multipart/alternative attachment
An HTML attachment was scrubbed...
URL: https://www.moypiano.com/ptg/pianotech.php/attachments/3e/60/81/1e/attachment.htm
---------------------- multipart/alternative attachment--
This PTG archive page provided courtesy of Moy Piano Service, LLC