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In a message dated 2/12/2002 11:52:14 AM Pacific Standard Time,
fordpiano@lycos.com writes:
Hey Phil
Excellent reply. Not being an expert on laminated ribs or any thing
else ,I stand corrected But, I do have a couple of possible complications to
the modulus of elasticity formulas and that is that the strength of woods
seem to vary between species and even in the same tree as you pointed out so
knowing how to select materials makes a huge difference when making
generalizations about the modulus of elasticity. Still agood place to start.
I've held Sitka panels for example that were flacid and flexible and another
much stiffer.This will make a difference in the ribs I choose for each board
.
I was thinking about your comments about strength uniformity of
laminatd ribs and I believe this must be the major benifit. However I think
it's possible to get some semblance of or closer to uniform results for rib
strength IF you can cut all your ribs from the same board. This is what I do
with the sugar pine ribs I use. I select it myself looking for a nice tight
grain12 to 15 grains per inch. Just look at new Stwys the're still using flat
sawn sugar pine that must be 3 grains per inch.How strong can that be? Sugar
pine is extremely variable.
As to the popularity of spruce ribs I think it is important to reiterate
my comments about how stiff is stiff enough in conjunction with what type of
soundboard material they're being glued to and at what grain angle. All
these factors add stiffness. I have used a variety of different types of
woods in different parts of the scale and have come some general tonal
conclusion on sound and not necessarily the modulus of elasticity. At this
point in time with Sitka panels I like all tight grain sugar pine ribs or
sugar pine in the top and some species of spruce in the bottom. Why because I
like the sustain qualitys. I have a light but stiff rib glued to Sitka which
has a stiffer strength to weight ratio than other species of spruce So I
don't really need extra mass in the panel. If I was going to use spruce in
the treble I believe wide grain sitka a better choice.
This may be a different opinion than many of my colleagues.
Also variables in soundboard construction methods contributes to a
prefrence too. If I'm using a softer species of spruce panels then spruce or
yellow pine seems to give a better result. Soo many variables.
One last comment about laminated ribs. It isn't just a glued up straight
beam it's a glued up curved beam. I believe in construction terms it would be
a prestressed beam or some such. This seems to me that it would make it
stiffer and more resistent to bending stress which would allow for a more
permanently crown.
I wish we could have discussed all this stuff back when I knew
everything!
>>>>>>>>>>>>>>Dale Erwin>>>>>>>
>
>
> Terry,
> In theory the bending stiffness of a beam (or in our case a rib) is
> dependent on
> the length, moment of inertia of the cross section, and the modulus of
> elasticity
> of the material. Assuming two ribs of the same length we can eliminate
> that
> variable. The moment of inertia is determined by the geometry of the cross
> section. So, two sections with the same cross section (for example a 1 x 1
> inch
> square) will have the same moment of inertia, whether that cross section is
> solid
> or built up of several layers. The modulus of elasticity is a physical
> property of
> the material. So, pine will have one modulus, spruce another, maple
> another, etc.
> Once again, the modulus of elasticity of the rib material shouldn't depend
> on
> whether the cross section is solid or built up in strips, as long as all
> the strips
> are of the same material as the solid cross section. That's the theory.
> In
> practice your mileage may vary.
>
> Here's a reference for some wood moduli:
>
> http://www.woodbin.com/ref/wood/strength_table.htm
>
> A couple of comments about this. Dale commented that spruce will give a
> stiffer rib than pine because it has higher stiffness. As you see from the
> chart, it depends on the type of spruce and the type of pine. Sitka spruce
> has a higher modulus than sugar pine, so in that case it's true. A sitka
> spruce rib of the same dimensions as a sugar pine rib will be stiffer.
> But as you can see longleaf pine has a higher modulus than any other
> softwood on the chart (however, it's also heavy). So, a longleaf pine rib
> of the same dimensions would be stiffer still. I believe the more
> important
> consideration for ribs, especially at the top end of the scale, is the
> stiffness
> to density ratio (from the chart the modulus of elasticity divided by the
> specific gravity).
> You want something that's stiff but light. Sitka spruce is very good in
> this
> regard, which is probably one of the reasons it has become a material of
> choice. However, as the chart shows, there are some others which are
> better.
> Red spruce, for example. Also, plain old doug fir (as long as you get a
> piece
> of coastal fir) is better still.
>
> The values given there are average values based on a large population of
> samples. Individual pieces of wood will of course vary. I'm sure it's
> possible
> to get a particular piece of sugar pine that is stiffer than a particular
> piece of
> sitka spruce and would have superior stiffness to density ratio. That's
> what
> selection of material is about. But the chart gives good general
> guidelines.
>
> I think there is something to the idea that the grain of a laminated rib
> will follow
> the line of the rib, rather than having the line of the rib cutting through
> the grain
> as in a solid rib. In theory it sounds better. In practice I'm not sure
> how much
> it matters.
>
> Phil F
> ---
> Phillip Ford
> Piano Service & Restoration
> 1777 Yosemite Ave - 215
> San Francisco, CA 94124
>
>
> On Mon, 11 Feb 2002 21:53:15
> Farrell wrote:
> >I guess a lot of my thinking comes from laminating ribs on boat hulls. I
> do realize that a soundboard is not a hull. And the laminated ribs on the
> boat replace steam-bent oak ribs. The laminated ones are nice because they
> tend to hold their shape way better than the steam bent ones. You may well
> be right that laminated would not be any stiffer than a solid rib. Part of
> my thinking is also that you need to saw through the grain to make a
> crowned soundboard rib - but of course, you are only cutting into it
> 1/8-inch or so. Some boats have sawn ribs and you end up with much of the
> wood grain not in a preferred orientation. I guess not such a big deal on
> the piano. I was also thinking of making a good use for the old soundboard.
> If you had laminated ribs, you could just cut up the old soundboard and at
> least make nice laminated ribs from it.
> >
> >I'm also just posting because no-one will help me with my German F. Weber
> serial #19290!
> >
> >Terry Farrell
> >.
> >> Terry,
> >> What makes you think that the laminated rib would be stiffer than a
> solid rib of
> >> the same dimensions (assuming the laminations are of the same material
> as
> >> the solid rib)?
> >>
> >> Phil F
> >> -
> >> >
> >> >
> >>
> >> .
> >
> .
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