[pianotech] Measuring Crown Radius

[Ron Nossaman]

On 7/12/2011 10:02 PM, David Love wrote:
> Just one point for now, so you're saying that the rib in a compression
> system offers nothing as a functional supporting member?

Yes I am. It supplies something for the expanding panel to pry against 
to provide crown and stiffness. It resists crown, and is functionally 
negative in beam support. The expanding panel has to not only bend the 
flat ribs into a crown, but support string bearing as well.


>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?

It doesn't, ever. That's why I said that.


>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?

No, it's only offering structural beam support if it's directly 
supporting load, as in a RC or RC&S assembly. CC ribs don't directly 
support load.


> That doesn't make sense.  A beam is a beam whether it's crowned or not.

Yes, it makes sense. Think about it. It's not the crown, it's the 
direction of load.


> 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.

Yes, but not as a load support beam. The panel is doing the work, and 
the CC rib is adding to the panel's load as the panel forces the flat 
rib into a crown. 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. This is the fundamental 
difference between CC and RC(&S) boards, which is why I say I find rib 
analysis of a CC board to be of no use to me. The ribs aren't supporting 
the load, so their dimensions don't tell me anything useful.

RC&S ribs (at least mine) are built as structural beams to support crown 
under bearing load as if the panel wasn't even there.

Ron N