dissent on compression -- revisited

[Delwin D Fandrich]

Dissent on Compression -- revisited.
On second reading, some additional thoughts come to mind:

Stephen Birkett wrote:

(snip, snip)

> ... The surface area of the board has increased as a consequence of
> differential drying, namely the drying of the board to an EMC much lower
> than will be experienced in ambient conditions....  the natural surface
> area of the board will increase as the board regains its moisture content.
> The final surface area will depend only on the relation between dry board
> EMC and ambient EMC.  The cellular and intra-cellular forces are
> compression forces, as the moisture content between cell walls increases.
> The cells will collapse if these forces exceed a critical value. The
> extent of the compression will depend critically on the degree of drying
> the board undergoes before gluing on the ribs.

If I'm not mistaken, this is pretty much what I've been saying all along.


> Compression is somewhat
> lower on the upper surface than the lower.

Theoretically, yes. But the differential would be very slight and would be pretty hard to measure.


> Finally there is the balance of forces achieved from the distributed
> down-bearing components once the board is installed. These act to
> decrease the curvature of the board, therefore reducing its surface area,
> and thereby increasing the compression further, from purely mechanical
> reasons.
>
> To achieve a balance that keeps compression below critical levels will
> require consideration of quite a few design parameters. From what I've
> seen, the dry-board EMC is rather critical and also rather difficult to
> control in typical manufacturing conditions.

Correct. And this is just one of the problems with this process.


> In addition, excessive
> downbearing forces may increase compression beyond material capacity.

Again, correct. Remember as well, downbearing forces change constantly as RH changes and EMC changes accordingly.


> My "theory" (if it is to be called that) is that careful consideration of
> these two factors can produce a purely compression crowned board with flat
> ribs on a modern piano that maintains compression within the critical
> material parameter for failure.

This is possible only if all parameters are perfectly controlled (or exceedingly close to it). There is no margin for error
at any stage. The process has to be perfectly controlled and the wood has to much closer to perfect than any of us have seen
for quite a while. In other words, while I might be willing to concede that it may still theoretically possible to do this,
but in practical production terms? Not likely.


> On the other hand, I would imagine that
> either, or both of: failure to maintain dry board EMC within specified
> manufacturing parameters, or poor design with respect to downbearing,
> might explain many observed soundboard failures. Those Grand Canyon ridges that have been mentioned from time to time on
> the list.

As I said above...


> Beginning with curved ribs will give the board a head start so to speak,
> since its "natural" condition after glue up involves a somewhat increased
> surface area, without taking into account the moisture-related surface
> increase. Therefore some expansion can be taken up without producing any
> compression at all, simply cancelling the tension from glue-up on curved
> ribs.

There is no "tension" involved in the process. The ribs are simply pre-crowned and pressed onto the panel which is already
curved to match by being placed on curved bottom cauls.

The advantage comes from not having to dry the board to such a low EMC prior to ribbing. We use 6.0% EMC, as opposed to 4.0%,
when the board is ribbed.



> I can't really see any advantage to not using curved ribs...

Nor can I.



> ...these were almost universally used on historical pianos with less down-bearing forces to boot. I suspect the reasoning
> is that a board already under extensive compression, which will be further increased by downbearing
> forces...

No, this is not the case. By not drying to panel as much, more of the crown is controlled by the curve of the rib. The amount
of crown to put in each rib must be carefully determined, of course.


> ... will take less time to "develop".

Not at all. It has been our experience that boards crowned by a combination of more moderate panel compression and
pre-crowned ribs are both more predictable and more stable than those crowned by compression alone.


> Meaning the time period for
> settling, as internal forces reach equilibrium, will be reduced. Faulty
> reasoning though if the board goes poof as a consequence. However for
> short term rental instruments it may be an expedient to avoid that
> waiting period for tone to develop. Trouble comes if they sit around too
> long in the warehouse.
>
> Stephen
>
> Stephen Birkett Fortepianos
> Authentic Reproductions of 18th and 19th Century Pianos
> 464 Winchester Drive
> Waterloo, Ontario
> Canada N2T 1K5
> tel: 519-885-2228
> email: birketts@wright.aps.uoguelph.ca

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There is really no need to compromise long life for short term performance. And I doubt that anyone does. At least not
deliberately.

Del