CA gluing Grand Pinblocks....My take on it/ Richard Brekne

[Farrell]

This is a multi-part message in MIME format.

---------------------- multipart/alternative attachment
Comments interspersed below:

Terry Farrell
 =20
----- Original Message -----=20
From: "Richard Brekne" <Richard.Brekne@grieg.uib.no>
To: <pianotech@ptg.org>
Sent: Wednesday, July 31, 2002 7:22 PM
Subject: Re: CA gluing Grand Pinblocks....My take on it/ Richard Brekne


> Joseph Garrett wrote:
> >=20
> > ! How, prey tell, did the bridge become cracked, in the first place?
> > answer: by the force of the piano wire trying to straighten out at =
the
> > bridge pins/bridge! If that ain't shear stress than I don't know =
what is!
>=20
>=20
> I guess this is where I am getting hung up here... I can
> easily accept that there is a shear force exerted on the
> bridge pin getting "pinched" as it were between the string
> pushing at it from the one side, and the bridge from the
> otherside.=20

I believe the above described a compression force of the string toward =
the pin, and a second compression force between the pin and the wood on =
the other side of the pin. If the pin had been glued in, then there =
would also be a tension force between the pin and the bridge wood =
directly under the string.

> What I dont see is that cracks in the bridge are
> caused by a shear force exerted on the bridge itself.=20

I don't imagine they would be. I can only imagine the crack being made =
by the pin migrating through the wood and pushing (compression) out =
perpendicular to its direction of migration, thus cracking the bridge.

> The
> force acting on the bridge here is the pin pushing against
> it perpendicular to the surface of the bridge. So the bridge
> feels a compressive force, not a shearing force.

Correct. BUT, if the pin were also glued to the wood in its hole, until =
the glue completely fails, there would be a shear stress between the pin =
and the glue and the wood on the forward and rear side of the pin (where =
the pin would be sliding past the stationary wooden bridge during its =
migration).
=20
> Terry, would you clear me up on exactly what directions of
> shear force you are refering too below in your first
> sentence. I can only imagine some twisting shear here as the
> strings are actually being brought up to tension. The rest
> of your paragraph makes perfect sense to me.
>=20
> Terry Farrel wrote:
>=20
> >When the piano is strung and the side pressure is applied to the pin =
by the string, you
> >will have a shear force between the pin and the maple on the inside =
of the bridge pin hole. >You will also have compressive forces applied =
to the CA that is filling the crack on the >side of the bridge pin =
opposite the string. As the bridge pin tries to move away from the =
>string, it will apply outward forces (tension force) to the CA layer =
and/or maple on the >inside of the crack (at the edge of the bridge pin =
hole) - trying to once again split the >bridge cap open.
>=20
> Cheers :)
>=20
> RicB

---------------------- multipart/alternative attachment
An HTML attachment was scrubbed...
URL: https://www.moypiano.com/ptg/pianotech.php/attachments/b4/81/ce/2b/attachment.htm

---------------------- multipart/alternative attachment--