bridge indents

[Delwin D Fandrich]

----- Original Message -----
From: "David Skolnik" <skolnik@attglobal.net>
To: <caut@ptg.org>
Sent: April 06, 2002 5:59 AM
Subject: Re: bearing, loose bridge pins leading to Wapin considerations


> Another question I asked which, thus far got no takers...about preloading
> the board before stringing.  If the cap is as impressionable as it seems,
> couldn't the initial tensioning of the strings cause the deep front
> indentations?



David,

I shouldn't think so.

Removing strings from a reasonably new piano--or newly strung--piano
generally reveals only a very slight indentation on the bridge surface;
regardless of how it is strung. And I doubt that many are pre-loading the
board before stringing. Which is not to say it isn't done, just that it's
not been common practice over the years.

Any increased indentation toward the front of the bridge is probably the
result of the string being trapped between the surface of the bridge and the
angled bridge pin while it's being dragged up to pitch. Pre-loading the
board will not alleviate this.

The cap, assuming it's made of some reasonably hard wood, is not all that
impressionable to short term loading of the magnitude in question. In
general, there are two possible ways to damage wood fibers--subject the them
to either a short-term, relatively high load or a long-term load of some
lesser magnitude.

First, short-term loading. Wood fiber damage--in this case, bridge surface
indentation--does not occur unless a load exceeds the compression strength
of the wood fiber with enough force to immediately and permanently alter the
fiber's shape. This would happen if a string were tapped hard enough to
press it against the surface of the bridge with enough force to bend or
distort the wood fibers beyond their fiber-stress proportional limit (fspl).
Once wood fibers have been strained beyond their fspl permanent deformation
results. In this case a permanent indentation will have been created.

Second, long-term loading. The other cause of surface indentations comes
from applying a long-term load or force to the surface of the bridge
resulting in the gradual, but permanent, deformation of the wood fibers
(creep). And, in this context, 'long-term' refers to a period of time
measured in years, not weeks or months. A load level insufficient to cause
permanent fiber deformation in the short-term might well cause significant
damage in the long-term.

In this I tend to agree with Ron (and others?) that the two primary causes
of bridge surface indentations are the excessive tapping (seating) of
strings against the bridge surface and the cyclic loading that is the
consequence of the strings being trapped between the angled pins and the
expanding (and contracting) bridge surface.

All of which is just one more argument in favor of epoxy-saturated laminated
bridges. Or at least something beyond today's traditional norm.

Del