Why piano tuning pin bushings?

[Delwin D Fandrich]

----- Original Message -----
From: "gordon stelter" <lclgcnp@yahoo.com>
To: "Pianotech" <pianotech@ptg.org>
Sent: October 24, 2002 3:02 AM
Subject: Re: Why piano tuning pin bushings?


>     I have a  feeling the ommission was meant to be
> tonal---some sense that some portion of the
> vibrational energy would pass through the string, into
> the block, back to the rim and belly rail and then on
> to the board without being absorbed by the plate. Cast
> iron has great compressive stength, and my impression
> has always been that plates are designed to take the
> full tesnsion, while the rim just keeps them bolted
> into a flat plane, so they do not twist and snap.

Well, not really and yes. Cast gray iron--the term 'cast iron' by itself is
meaningless--of the grade most commonly used in piano plates has relatively
moderate compressive strength and even lower tensile strength. The tensile
strength of the iron used in piano plates typically has a tensile strength
of between 15,000 and 25,000. The iron in some early plates yielded an even
lower tensile strength. Gray iron gained its reputation for 'great
compressive strength' because its compression strength is higher--two or
three times higher is not uncommon--than its tensile strength. With other
iron alloys and steel these are either the same or close to the same.

And yes, the plate should be designed to support all, or at least most, of
the stress load impressed on it from the strings. The rim and/or back
assembly acts as a stabilizer. (Obvioulsy, the plate horn and wedge
mechanism transfers some of this stress to the belly assembly in those
pianos that use them.)


>
>      The only significant structural difference I
> sense is that a bushing would put the fulcrum of
> twisting force caused by string tension higher up,
> rather than from beneath the plate.

In the end the stresses on each system will be about the same. Of more
importance is how high the string termination on the tuning pin is above the
speaking string plane.

Del