String vibrations (was Re: Stuart)

[Delwin D Fandrich]

----- Original Message ----- 
From: "Overs Pianos" <sec@overspianos.com.au>
To: "Pianotech" <pianotech@ptg.org>
Sent: September 23, 2003 12:41 AM
Subject: Re: String vibrations (was Re: Stuart)


>Two things worry me about using bridge agraffes (I admit that I am
> interested in using them at some point).
>
> The first is that the agraffe takes up valuable bridge height. If,
> for example, the string plane is say 8 mm higher than the bridge on
> account of the agraffe taking up the space, a piano with such an
> agraffe will have only 24 mm high bridges if the string plane is 32
> mm above the panel. I consider 24 mm as an inadequate height for the
> long bridge, and all pianos which I have so far experienced with such
> low bridges seem to exhibit tonal problems. Therefore, the bridge
> agraffe should be designed with the lowest practical height between
> the top of the bridge and the string plane. Alternatively, one could
> build a panel with another bridge under the sound board as Grotrian
> did with some of their early grand pianos, but this would push the
> cost of the piano up quite a bit.

There is no immutable law that requires the strings to be 32 mm above the
soundboard surface. If this is a concern -- with a new design in any
case -- the string plane can simply be moved up a few mm. Another way to
cope with the problem is to take the route Stuart followed and use a whole
lot of ribs. The bridge is, after all, a bridge between the ribs. The
shorter the span between the ribs the less absolute stiffness is required
to bridge between them. This practice, of course, while solving one problem
introduces several of its own.


>
> The second concern I have with using bridge agraffes is fixing them
> permanently to the bridge. Of the bridge agraffed pianos I have
> heard, I suspect that this can be a problem. It may be possible to
> achieve a good connection by using West system glue between the
> agraffe and the bridge, and using machine screws to secure the
> agraffe to the bridge, while at the same time gluing them into the
> bridge with West system. Stephen Paulello's agraffes are glued to the
> bridge. Take a look again at the following close-up image of his
> agraffe. A light blue adhesive can be seen at the junction of the
> agraffe with the bridge.

This was the major concern at Baldwin. We did not actually encounter any
such problems but several thought there might be on down the road. We
investigated bonding them to the bridge surface but there were a number of
concerns with this. Any adhesive used must allow for wood movement against
a stable metal piece. It must also be vibration tolerant. Both of these
requirements ruled out CA adhesives. At least at the time. Considerable
progress has been made with CA adhesives in subsequent years. Since a major
feature of the device was ease of manufacturing it had to go on fast. This
ruled out epoxies. Again, at least at the time. There were a couple of
modified acrylics that held promise but they were expensive and messy.
Automatic application equipment might have eased the mess but not the cost.
Any adhesive that formed a compliant film -- PVA, etc. -- was ruled out for
obvious reasons.

Del