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

[Farrell]

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Comments below:

Terry Farrell
 =20
----- Original Message -----=20
From: "Richard Brekne" <Richard.Brekne@grieg.uib.no>
To: <pianotech@ptg.org>
Sent: Wednesday, July 31, 2002 1:36 PM
Subject: Re: CA gluing Grand Pinblocks....My take on it/ Richard Brekne


> Farrell wrote:
> >=20
> >I should think that a cracked bridge pin hole repaired with CA (or =
any other adhesive) >would be subject to shear, compressive and =
tensional forces. 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.
>=20
> Not sure I follow here... why would side pressure on the pin
> by the string result in a shear force... ???

Boy, a picture would be worth a thousand words. I am just talking about =
a pin in a hole in maple. Push it to one side. You will have tensional =
forces between the pin and wood on the side of the pin where the force =
was applied. You will have compression forces 180 degrees to the tension =
forces. And you will have shear forces between the pin and the wood =
along a plane parallel to the pushing force (90 degrees from the =
compression and tension forces).

> I see the pin
> being pushed away from the string in a sideways direction
> with the wood of the bridge on the opposite side of the pin
> resisting... ie tension force mainly.=20
> Since the string is "bent" over the bridge to begin with I
> dont see much upward force on the pin, and unless the string
> is moveing perpendicular to the top of the bridge (as in
> tuning) I dont see any turning force either. Arent these the
> only two shears possible between the two sufaces (pin and
> hole) ?
>=20
> >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
> I see this alright... seems like the brother part of any and
> all forces being applied here. Perhaps this is a
> misconception ??
>=20
> =20
> > I would think that a CA repair on a bridge with loose pins would =
likely be a reasonable >approach (mostly just compression forces), but I =
don't think I would be comfortable >repairing a cracked bridge with CA. =
I'd break out the West System epoxy.
> >
>=20
> That seems pretty reasonable on the surface of it... tho I
> have even run into a fellow who repairs cracked pinblocks
> using a combination of CA and CA exciter, and he swears by
> this method. He is a fellow who's education and reputation
> make it rather difficult to simply dismiss his ideas.
>=20
> RicB
>=20

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