<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML><HEAD> <META http-equiv=Content-Type content="text/html; = charset=iso-8859-1"> <META content="MSHTML 6.00.2600.0" name=GENERATOR> <STYLE></STYLE> </HEAD> <BODY> <DIV><FONT color=#008080>Comments interspersed below:</FONT></DIV> <DIV><FONT color=#008080></FONT> </DIV> <DIV><FONT color=#008080>Terry Farrell</FONT></DIV> <DIV> <SPAN id=__#Ath#SignaturePos__></SPAN> </DIV> <DIV>----- Original Message ----- <DIV>From: "Richard Brekne" <<A href="mailto:Richard.Brekne@grieg.uib.no">Richard.Brekne@grieg.uib.no</= A>></DIV> <DIV>To: <<A = href="mailto:pianotech@ptg.org">pianotech@ptg.org</A>></DIV> <DIV>Sent: Wednesday, July 31, 2002 7:22 PM</DIV> <DIV>Subject: Re: CA gluing Grand Pinblocks....My take on it/ Richard Brekne</DIV></DIV> <DIV><BR></DIV> <DIV>> Joseph Garrett wrote:<BR>> > <BR>> > ! How, prey = tell, did the bridge become cracked, in the first place?<BR>> > answer: by = the force of the piano wire trying to straighten out at the<BR>> > bridge pins/bridge! If that ain't shear stress than I don't know what = is!<BR>> <BR>> <BR>> I guess this is where I am getting hung up here... I can<BR>> easily accept that there is a shear force exerted on = the<BR>> bridge pin getting "pinched" as it were between the string<BR>> = pushing at it from the one side, and the bridge from the<BR>> otherside. </DIV> <DIV> </DIV> <DIV><FONT color=#008080>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.</FONT></DIV> <DIV> </DIV> <DIV>> What I dont see is that cracks in the bridge are<BR>> = caused by a shear force exerted on the bridge itself. </DIV> <DIV> </DIV> <DIV><FONT color=#008080>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.</FONT></DIV> <DIV> </DIV> <DIV>> The<BR>> force acting on the bridge here is the pin pushing = against<BR>> it perpendicular to the surface of the bridge. So the bridge<BR>> feels a compressive force, not a shearing force.</DIV> <DIV> </DIV> <DIV><FONT color=#008080>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).</FONT><BR> <BR>> Terry, would you clear me up on = exactly what directions of<BR>> shear force you are refering too below in = your first<BR>> sentence. I can only imagine some twisting shear here as the<BR>> strings are actually being brought up to tension. The = rest<BR>> of your paragraph makes perfect sense to me.<BR>> <BR>> Terry = Farrel wrote:<BR>> <BR>> >When the piano is strung and the side = pressure is applied to the pin by the string, you<BR>> >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.<BR>> = <BR>> Cheers :)<BR>> <BR>> RicB</DIV></BODY></HTML>