laminated ribs

[David Love]

One question I have is why, if you want to create a higher design loaded
board, do you need to increase the bearing?  On an RC&S board, with little
change in tone coming from modest changes in bearing, isn't it enough to
simply stiffen the assembly with the same residual bearing?  The difference
would be, I assume, the amount of deflection.  But if the panel deflects 30%
instead of 50% is that a problem tonally?  Wouldn't you have the same net
tonal effect without having to increase the actual bearing load on the
board?  

David Love
davidlovepianos at comcast.net 

-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Ron Nossaman
Sent: Saturday, March 25, 2006 10:16 AM
To: Pianotech List
Subject: Re: laminated ribs


> I see.  By larger piano do you mean a D?  Or are you going up to 900 lbs
> for, say, a B as well?  

Thereabouts, possibly a bit less. I don't have hard rules, 
because the methods are evolving.


>You mentioned previously that you are building the
> ribs for higher load than you started originally and you found that there
> was some relationship to hammer tolerance.  The boards that you are now
> building that are set up for, say, 650 lbs, what would those have been
> previously?

Probably not much different, but I can't say for sure. This 
isn't a very thoroughly documented research attempt, and I 
tend to pay closer attention to proportional cause and effect 
relationships than to specific numbers. I don't start out with 
a specific design load target anyway, and it ends up where it 
ends up when I'm reasonably satisfied with the design as to 
how it looks in the spreadsheet, how it is expected to perform 
and what it will teach me through the project. I'm making the 
ribs (therefor the assembly) generally stiffer, and adjusting 
the remaining crown up. The postmortem has indicated that the 
result is a greater tolerance, but not a requirement of harder 
hammers. This connects, to me, with the very high spring rate 
of a new CC or RCPS board and it's need, not tolerance, for 
harder hammers.


> Just as an aside, you said that you plan for 40% deflection and end up
> somewhere between 40 and 50%.  I assume you are calculating your
deflection
> from beam formulas and basing it on the rib properties only and ignoring
the
> panel in the calculations, for the most part.  

Right.


>It's interesting that when
> you calculate for deflection based on the ribs only that the panel itself
> contributes very little if anything to the stiffness of the assembly in
> terms of the overall deflection.  Do I understand that correctly?
> 
> David Love

Apparently so. The panel does contribute to overall stiffness, 
just not much in terms of deflection. Like that photo I posted 
a couple of weeks back of a nearly 3mm residual crown in the 
killer octave of a board with the strings on and up to pitch, 
with close to 1.5° of bearing at that spot, and the panel MC 
slightly below that at which I ribbed it.

Ron N