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

[Richard Brekne]

Joseph Garrett wrote:
> 
> ! 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!


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

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.

Terry Farrel wrote:

>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.

Cheers :)

RicB