[pianotech] Measuring Crown Radius

[David Love]

Just one point for now, so you're saying that the rib in a compression
system offers nothing as a functional supporting member?  Why?  So let's
assume a rib in that system is glued to a panel at 4.5% EMC.  At that level
you say it's not a functional supporting member.  So then when does it
become a functional supporting member?  At 5%, 6%, 7%?  Are you saying it's
only a supporting member if it's crowned and glued to panel at 6% or higher?
That doesn't make sense.  A beam is a beam whether it's crowned or not.
It's relative strength may be influenced some be crowning (don't know about
that), by tapering at the ends (a feature that ribs in RC&S boards also
have) but it still functions as a beam.  

So the quantifiable analysis of ribs in compression crowned systems is there
for the taking just as in any other system.  There are certainly other
factors in play and they certainly vary between systems and designs but that
doesn't redefine it as something other than a beam.  If it does then it
suggests that RC&S systems have crossed over into another universe.   

David Love
www.davidlovepianos.com

-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Ron Nossaman
Sent: Tuesday, July 12, 2011 7:13 PM
To: pianotech at ptg.org
Subject: Re: [pianotech] Measuring Crown Radius

On 7/12/2011 8:02 PM, David Love wrote:

> Once glued into the rim the rib becomes a clamped ends beam and even
> if it is not a buttressed arch, gluing it in affords it some
> additional resistance to bending.

Yes it does, but the thin rib ends don't supply much in the way of 
support, which is why measuring unloaded crown is of so little value.


>If that's the case then an
> analysis of the rib structure even in a compression crowned system
> could be useful and even if it's not that useful it seems to be
> telling.

What analysis? There's nothing there that's quantifiable.


>If one assumes on a rebuild using the original board that
> the ribs may have to do some additional work then knowing whether the
> rib scale is not up to the task or is imbalanced in terms of its load
> carrying capacity from rib to rib, then I would find that useful
> information.

If one assumes so, yes. I don't. One can imagine anything.


>As I do tend to run rib scale analysis on existing
> boards (even compression crowned boards), it's interesting not only
> the variation between the same models.  Also, I find, that weaknesses
> in certain sections in the array are fairly evident when you put
> numbers to them.  Often those weak areas correspond to problem areas
> in the piano.  Compression or not, it seems worth paying attention
> to.

I haven't found it useful at all, since CC ribs aren't functionally load 
supporting members, but rather primarily constraints to panel expansion.


> We may disagree here then.

Not may, we do. I just said so.


>The assumption is that the original load
> was set up properly or that after the fact someone under read the
> desired load and increased it by pulling down the nosebolts or
> lowering the plate (if the piano had that system).

No, I make no such assumptions. I try to get a non prejudiced picture, 
without guessing, of what the board is capable of, and the balance 
across the scale, by the interactions of crown and bearing throughout.


>One could find
> (it doesn't happen that often but it does happen) that the piano had
> adequate crown but that the bearing was set too high.  Even if the
> loaded readings of crown and downbearing were not desirable, the
> crown coming up with the release of tension to adequate levels and a
> showing that the bearing was set too high might lead you in the
> direction of not replacing the board when you otherwise might have.

That's one reason I find no real value in unloaded crown measurements.


> Yes, you did answer the unloaded crown question and I should have
> worded it more clearly.  Given that you normally set up a board with
> a standard load of, say, 700 lbs then you're right the before and
> after measurements will tell you if the board will take that
> load--assuming everything goes back to where it was before you took
> it apart, which it doesn't always but that's another issue.

I don't have a standard load for an existing board. If I'm keeping the 
original board, the loading will be determined by what the crown and 
downbearing measurements indicate. What before and after measurements?


> So my question
> was more what would be a minimal load setting that would likely
> produce an acceptable, albeit not an ideal outcome.

That's unanswerable. It depends on what's there, and the situation.


>Related to that,
> what would be the minimum crown required in order to do that.  I
> guess that's really the secondary question as it's easy enough to
> figure out the maximum bearing you can set on a given piano with a
> certain amount of crown  so that you avoid the risk of pushing the
> board negative.

And that maximum bearing determination will also tell you if the maximum 
is minimal, in which case you're on the edge if you proceed. Also, in 
the land of seasons, we have a little situation that the west coasters 
don't apparently have to deal with. A soundboard that sounds fine in 
June, with not great but minimally adequate crown and bearing 
measurements, can often sound terrible in January when everything dries 
out and the panel loses it's wet season compression.


> But how much load is required to produce an acceptable sound is my
> question, and that's assuming that the load can be set uniformly
> which it can't always in part due to poorly designed rib scales.

Again, unanswerable. It depends on the particular instance.
Ron N