RC vs CC again

[Ron Nossaman]

>If you cut a rib from a Panel crowned soundboard (parallel to the
>rib but half way between the adjacent ribs) you have a laminate made
>from a rib and a strip of cross grain spruce. The two pieces of wood
>form their crown just the same as the above laminated rib. Once formed
>this structure is going to behave just the same as if the crown were
>formed from a crowned rib.

This doesn't equate in the real world with actual wood. Wood is 
anisotropic, and compresses considerably more across the grain than it does 
along the grain. It takes considerably more dimensional change in a cross 
grain spruce panel to generate the cross grain compression levels it takes 
to both bend the rib (which, itself is not made of multiple layers formed 
and glued, but is one solid piece that resists bending far more than a 
stack of laminations of the same overall depth), and support the bearing load.


>I admit that it took force to form the crown in a panel crowned
>Soundboard but once formed it will have just the same stiffness as a Rib
>crowned soundboard. The method of crowning has no effect on its stiffness.

We aren't talking about stiffness, but about the panel supporting both the 
string bearing load, and whatever crown the rib is forced into.


>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.

Additional 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.


>  .002" compression (strain) of a cross grain
>strip of spruce say 48" long will indicate the same stress whether it
>started in a stressed state or not. Stress/strain remains proportional
>if measured in inches per inch (percent) and if kept below the plastic
>limit (the compression of the panel of a PC board is below this limit).

Not possible. The total compression stress will be whatever it takes to 
bend the ribs added to whatever it takes to support bearing, not just the 
0.002" difference between crowned and forced flat. Saying that the board 
isn't under stress by being constrained enough to bend the ribs doesn't 
make it so. 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. 
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. 
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.

Ron N