capo-hardening/string breakage

[Robert Hohf]

There's something about this capo string breakage that I haven't seen
mentioned here yet. This isn't the one answer to every case, but I've seen
this condition in two Ds that I've rebuilt. It's worth checking out
especially when the replacement stings keep breaking, too.  The machined
round surface on the bottom of the capo that provides the termination has an
"edge" on both sides.  Sometimes the round surface doesn't extend far enough
around the bottom of the capo, and the strings terminate on the edge rather
than tangentially to the rounded surface.  Obviously, this situation will
create a very abnormally high stress right at the edge that will resist
string rendering and greatly accelerate the process that causes the strings
to break.  With the strings out you can see exactly where the stings
terminate by looking carefully at the grooves, with a magnifying glass if
necessary.  The solution is to reshape the bar and remove the edge.

Bob Hohf
NOU

----- Original Message -----
From: "Fred Sturm" <fssturm@unm.edu>
To: <caut@ptg.org>
Sent: Thursday, October 11, 2001 11:28 AM
Subject: Re: capo-hardening/string breakage


> Richard,
> I'm just not really convinced that hardness per se is an issue for
> string breakage (at least within normal parameters: hard enough not to
> deform too much over time, probably softer than the wire itself - though
> even here I'm not entirely convinced). Seems to me this might be one of
> those areas where we assume we know something and then never bother to
> question it.
> Breakage of a string at the capo happens mostly from an action similar
> to kinking and rekinking a wire. THe bend is accentuated when the string
> is struck, then springs back beyond rest point - and back and forth
> during vibration. As long as the fulcrum (capo) is stable and firm -
> that is, it doesn't move significantly realtive to the string, and
> doesn't have "springiness" - why does it make a difference exactly how
> hard it is?
> I don't think the wire is deformed in cross section ("smashed") by
> hammer blows. In the first place, the hammer blows occur away from the
> point where the wire crosses the capo. In the second place, the hammer
> surface is springy. If it were a matter of deforming/thinning the wire
> during impact, hardness of the capo would matter. But I don't think the
> mechanics are set up in a way that would make this a factor.
> Sharpness of radius and high angle of deflection are very definite
> factors in breakage, no question. I just raise the question whether
> hardness per se, other factors being equal, has actually been shown to
> have a significant impact. Preferably experimentally. Can anyone point
> me to experimental data? Or offer a mechanical engineering explanation?
> No question that much work has been concentrated on this area of piano
> design. THis is the area pianists need to make "sing," to bring out
> above the rest of the piano. Hence duplexes, aliquots, unequal unison
> lengths, varying angles of deflection, etc, etc. Hardness/softness, as
> far as I have been able to determine has mostly been an issue of
> maintaining a clean termination over time. That is, at least, how I have
> always viewed it.
> Fred Sturm
> University of New Mexico
>