Aha! was flat facts

[Sarah Fox]

Hi Ron,

> There have been experiments, including making pins so tight in
> waterproof pinblocks that you could barely turn the pin in with
> three hands. Guess what. The super tight waterproof block pianos
> still dropped in pitch over the long run. In my experience, tuning
> stability gets better with decent scaling, rib supported boards, and
> laminated bridge caps. Hence my answer.

But has anyone ever looked for rotation in the pin?  That is, were pin
positions accurately measured/recorded, so that any migration in pin
position (corresponding to magnitude of pitch drop) could be measured?  If
not, then the question wasn't answered.

I can drive a 10 penny nail into a block of wood, and given conformational
cycles in the wood, due to cycling humidity, etc., the nail can walk out
when under no outward tension.  I'm not necessarily saying it does, but that
it can.  I think you will agree that the fit of a nail in a block of wood is
pretty tight!  When there are repeated changes in conformation of one
material against another, frictional effects are negated.  All that is
really required for the materials to walk is for the conformational change
to be great enough to result in slippage.  Now, if the pinblock cycles in
thickness enough that there is the teensiest bit of slippage of pin against
pin block in the longitudinal direction (and expanding wood can generate
some pretty impressive forces!), the pin will start walking, bit by bit, in
the direction of any forces applied to it, no matter how high the friction
of the pin against the pinblock.

The question of a moisture-impervious pinblock material is more interesting.
However, I would suggest that many mated materials are going to shift
against each other without the help of cycling humidity.  For instance,
there is the issue of thermal expansion.  One material may expand more than
the other when heated, hence slippage.  There's also the effect of
mechanical forces acting upon the two materials.  As an assembly is torqued
and vibrated back and forth, the parts can walk against each other.  There
may even be stochastic gremlins at play. Perhaps on a molecular level there
are local, random conformational changes in the cellulose, as it mates with
the metal of the tuning pin.  Again, with time...

Other examples of slippage can be seen in machinery.  If two parts don't
corrode together, if they aren't "stuck" together (e.g. with LocTite), and
if they aren't locked with some sort of pin or lock washer (that digs into
the metal), they can walk apart, even when they are first assembled under
great tension.  Fact.

On a piano, the screw stringer design seems the most unlikely to walk.
Inquiring minds still want to know...  ;-)

Peace,
Sarah