[pianotech] Bridge agraffes FYI

[Encore Pianos]

Here is another issue with these agraffes.  If you look at the second
picture which gives you a little bit of a side view, you will see that the
strings are wanting to pull the agraffe up from the front of the  bridge
whilst pushing down on the wood cap in the back.  (The tension of the
strings is such that the wire is always trying to straighten itself.)  These
would seem to be strong forces to introduce bridge roll, although in the
opposite direction from what we are accustomed to with traditional notched
and drilled bridges.  Indeed, it is common to my experience to see the
traditional bridge cap which is used in the top section of the treble bridge
coming unglued and lifting away from the root at the lower end where it
meets up with the agraffed portion.  (The forces of each type of bridge are
applied in opposing directions)   Also, I see the type of cracking and
lifting away of the panel from the ribs in the agraffed portion of the board
that we associate with bridge roll.  Again, I have seen this pattern of
deterioration on enough of these puppies to believe that it is common to the
design.  

Will Truitt

-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of David Love
Sent: Thursday, October 25, 2012 3:22 PM
To: pianotech at ptg.org
Subject: [pianotech] Bridge agraffes FYI

  
Attached are two photos of a Sohmer Grand bridge with bridge agraffes.  Very
interesting in that the string bears on the top of the aggraffe hole, not on
the bottom as one might expect.  In order to maintain positive downbearing
on the bridge, the bridge has a raised shelf behind the aggraffe such that
the string runs uphill to the bridge from the hitch pin area before running
downhill to the aggraffe from the short span off the front of the shelf.
The slope of the string then rises as you move toward the tuning pin
termination side.  Sadly, I did not have my bubble gauge to try and
determine the net bearing and it's definitely got me reaching for the fish
oil capsules thinking about whether a measurement of the relationship
between the hitch segment and the front segment would reveal the net bearing
anyway.  The piano sounded like caca, btw, but there were other issues.  In
spite of that, the tone was surprisingly focused.  

David Love