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

[Keith Roberts]

Richard,  the shear force of scissors is two forces that are parallel but
opposite and they shear the hair off which is at 90 degrees to the forces.
Nail two blocks together and move them sideways and the nail will shear off.
The shear strength of the nail is at 90 to the nail. Shear wind is at 90 to
the direction of travel of the airplane. The string pulling the pin sideways
trying to cut the top off the pin could be called a shear force. Hard to
explain but did I make it clear enough?
Keith R
----- Original Message -----
From: "Richard Brekne" <Richard.Brekne@grieg.uib.no>
To: <pianotech@ptg.org>
Sent: Wednesday, July 31, 2002 10:36 AM
Subject: Re: CA gluing Grand Pinblocks....My take on it/ Richard Brekne


> Farrell wrote:
> >
> >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.
>
> Not sure I follow here... why would side pressure on the pin
> by the string result in a shear force... ??? 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.
> 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) ?
>
> >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.
>
> I see this alright... seems like the brother part of any and
> all forces being applied here. Perhaps this is a
> misconception ??
>
>
> > 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.
> >
>
> 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.
>
> RicB
>