[pianotech] Measuring Crown Radius

[David Love]

Ron:

You wrote:

"The force on a CC board rib is reversed from that on 
RC and RC&S ribs. It's still a beam, It's just not doing a thing to 
support crown and bearing other than constraining the panel. It's 
actually trying to pull the crown flat."

If you are arguing that the compression bent rib contributes negatively to
the load bearing properties of the panel by trying to pull the panel flat
then it follows that a lighter rib that pulls down less would contribute
less negatively and the load bearing properties of the assembly would be
increased. It doesn't take any significant depth of the rib to constrain the
panel on that side, you can do that with fairly thin piece of veneer glued
to the panel.  My example of an unribbed panel was extreme, of course, in
order to make a point.  I followed that with something more realistic when
discussing it in terms of a lighter rib.    

I'm questioning your basic premise that a bent rib once glued into the rim
is actually pulling the panel down and doesn't contribute to load bearing
support.  It's an important point because your claim is that the two systems
are diametrically opposed.  I'm arguing that they are the same system
essentially and the difference is only a difference of degree, which part of
the system does more work.  One can then reasonably argue longevity issues
associated with that.  The acoustical properties are another issue which is
outside the scope of this discussion for now.  But that's in important
distinction.  The fact is that all ribs are bent by compression some even in
RC&S boards--at least while the RH is above the EMC level at which they are
ribbed.  So then all ribs, by your account, are pulling the panel downward
and there's no real difference between the two systems except for one of
degree.  I'm open, show me the evidence or the engineering that says that a
bent rib when fixed at the ends in this case have no load bearing properties
or negative load bearing properties.  Because what you're suggesting is that
if you simply bend a rib and wedge it into the rim so that it is bent
upwards in the middle that it won't stay bent because it doesn't have the
panel holding it up. Well, that just doesn't happen.  

There's something else as well.  After the bent rib is glued into the rim
the string load compresses it further and the rib starts to push back up
against it.  The other observation which can be made is that you have panels
that are real toast, cracks, fissures, ridges, not likely capable of
supporting much of anything yet some of the panels still show some crown.
It's either that the panel is still supporting some crown or that the ribs
having been affixed to the rim after being bent are or some of both.  I
would argue it's some of both but at that point in the panel's life it's
probably not doing much.  

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: Wednesday, July 13, 2011 12:18 PM
To: pianotech at ptg.org
Subject: Re: [pianotech] Measuring Crown Radius

On 7/13/2011 1:58 PM, David Love wrote:
> I know that argument has been made but unless someone can actually show me
> how a panel crowned by simply gluing it into the rim without any ribs
> attached at a low EMC that causes it to crown up will be structurally more
> capable of load bearing (let's put stability aside for a minute) than one
in
> which ribs are attached and caused to bend by panel compression prior to
> gluing it into the piano then I don't buy it.

Jeez man, what the hell are you drinking? No one that I know of has ever 
claimed anything remotely like that. Read the posts. Read what they say. 
There's nothing like that here.


>Because that's the
> implication, that the unribbed panel when compression crowned will have
> superior load bearing properties to the ribbed panel, or, more reasonably,
> that a compression crowned panel with 5 mm tall ribs (let's assume the
same
> width) rather than 20 mm tall ribs will have more load bearing capacity
> because the ribs will contribute less negatively.

Again, no one has ever suggested an unribbed panel in any capacity. READ 
THE POSTS. I can't explain anything at all to you if you invent 
implications instead of making an effort at understanding the basic 
mechanics.

Ron N