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

[Farrell]

No sweat Richard. Let's picture a kinda funky bridge pin. It is a pin that has a square cross section and is 1 cm on each side (patent pending). Drill, er, a, mortice (is that how you make a square hole?) a nice square bridge pin hole for it and install. The pin has four sides: the string side; the side opposite the string; the forward side; and the rear side. 

Lets say the pin has been installed for a few years and you can pull it out with your fingers. String up the piano. The string will push on the square bridge pin and the pin will have a compression force exerted on the bridge wood on only the side opposite the string.

Now take the strings off, and glue the square bridge pin in place. Install strings. Now the string will push on the pin in the same way. But because the glue is bonding the pin to all four sides, there will be less compression on the opposite side, there will be a tension stress on the string side of the pin (the wood & glue under the string are trying to hold the pin in place), and there will be shear stress on the forward and rear of the square bridge pin (just like the two blocks of wood that are nailed together in previous examples - only we have glue instead of a nail).

All this will happen with the round bridge pin also, but it is not as easy to separate the vectors on a round pin. Ron was correct to point out that these other forces/stresses will be present when the pin is glued in place because other areas of the bridge are trying to hold the pin in place with the glue, but once the glue bond fails, then it is just the loose pin that is getting smooshed against the hole side opposite the string.

Actually, if the pin has a tight fit in the hole with no glue, there will be shear stresses also (friction of the pin against the wood on the front and rear side of the pin), but they will presumably be relatively low.

Is that better? I'm pretty sure I'm pretty close with this stuff.

Terry Farrell
  
----- Original Message ----- 
From: "Richard Brekne" <Richard.Brekne@grieg.uib.no>
To: <pianotech@ptg.org>
Sent: Thursday, August 01, 2002 5:05 PM
Subject: Re: CA gluing Grand Pinblocks....My take on it/ Richard Brekne


> Terry, and others following along.
> 
> After reading back and forth a few times I find myself in
> agreement with you on everthing except this one point about
> what happens when glue comes into the picture. You seem to
> be saying that when there is glue present between the bridge
> pin and hole then some shear stress is present that isnt
> there without the glue present. A couple posts back you said
> :
> 
> "..., 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)."
> 
> Thats where I get confused. The direction of the force from
> the string is the same, as is the resistance from the
> bridge. The pin still stands more or less perpendicular to
> these. The only thing that has changed is a thin layer of
> glue between the lengthwise surface of the pin and the
> inside of the bridge pin hole.  I think you are saying that
> at the very top of the hole, half the pin/glue/wood surface
> is under shear stress, and the other half is under
> tension/compression,,,. But if this is so, how significantly
> does this change the total picture, and how does any of this
> figure into bridge cracking ?
> 
> Just about there methinks :) I think I read you clear now on
> the rest of it.
> 
> Thanks for your patience Terry. :)
> 
> RicB