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--
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