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

[Farrell]

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. 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 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.

Terry Farrell
  
----- Original Message ----- 
From: "Richard Brekne" <Richard.Brekne@grieg.uib.no>
To: <pianotech@ptg.org>
Sent: Wednesday, July 31, 2002 7:26 AM
Subject: Re: CA gluing Grand Pinblocks....My take on it/ Richard Brekne


> Joseph Garrett wrote:
> > 
> > Richard,
> > Your statement: "And in the case of the bridge pin there is nothing to exert
> > any shear force on the CA itself so it should just......."
> > NOT! 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 understood shear stress to be stress applied parallel to
> the sufaces involved. In the case of bridge pins that would
> require either a force applied to twist the pins (as a
> tuning pin turns in the pin block) or a force pulling or
> pushing the pin up or down the length of the bridge pin
> hole. The force you refer too I understood to be more a side
> ways presure exerted from the string.... which would be more
> perpendicular to the hole eh ?? Perhaps some amount of shear
> stress might be applied from the strings pulling on the pins
> as the string moves under tension adjustments....but....
> 
> I thought bridge cracking was a caused mainly by a
> combination of climatic problems and side ways pressure on
> the side of the bridge pin hole opposite the strings ??
> 
> Please correct / expand if this is in error :)
> 
> > I just made a bridge for a tech, who used CA to "fix the cracked bridge". He
> > didn't like the look of it and sure as hell didn't like the sound.....So,
> > New Duplicate Bridge. I won't tell you what it cost, but I ain't a cheap
> > date.<G> Actually, the duplicated bridge was better than what you would see
> > in a top notch grand and this was for a Chickering Upright. I know from long
> > experience, that that bridge will meet/exceed the sound quality of the piano
> > when it was new. Not braggin', just have found this to be the case.
> 
> Braggin is cool.... grin at least the way you do it  ! In
> any case I believe you. Next time I am over there you can
> show me your stuff eh ?? My Dad lives in Salem.
> 
> > Best Regards,
> > Joe Garrett, RPT, (Oregon)
> > 
> > I'm never lost, because everyone is telling me where to go. <G>
> 
> Cheers !
> 
> RicB
>