Download PDF - Piano Technicians Guild

An Analysis of a Broken Plate
Continued from Previous Page
pounds upward at an angle
of 17.5 degrees to the rear.
The component forces on
the capo bar would then
have been 8000 pounds
straight up and 2650
pounds toward the rear.
Figure 4, attached,
shows the general pattern
of stress in this division of
the capo bar. Forces
pressing upward and
outward against the curve of the bar would create “stress
risers” at the bottom of each end. Tensile strains (for which
cast iron is weak) would then focus at these points and tend
to tear the bar apart here.
When the tensile strain exceeded the low elastic limit of
the casting at the right end of the bar, it broke. The fracture
appears to have begun at the bottom, near the front, and
moved upward in a matter of a few microseconds until the
32 Piano Technicians Journal / November 2000
Figure 4
break was complete; the
right end of the bar flew
up an inch or two, and
then fell back down to
where it is now, as the
plate cracked at the other
end of the bar due to the
sudden prying action of
the bar as its right end flew
upward.
In my opinion, the full
88-note capo bar in this
piano was an inexpensive and inferior substitute for much
better construction. The long curved span of the capo bar
was unrestrained in the middle. The string bearing-angle
was excessive. In my opinion, this bar was strained to its
limit as soon as the piano was tuned at the factory. It held
for the first few years, and then for the final decade of the
piano’s life while the string tension slowly decreased as the
piano was neglected and allowed to go flat. Then, when the
piano was finally tuned back up to
standard pitch, the weakened bar
broke.
Conclusion
In my opinion the plate in this piano
broke as a result of inferior and faulty
design. It would have broken, no
matter who tuned the piano. Fortunately,
piano plates rarely break, but
when one does, it is not uncommon
for it to do so some years after the
piano was built, and the cause is almost
always a fault of some sort in the plate.
Stress fatigue in metal does occur with
the passage of time, and microscopic
cracks do grow until the part finally
fails. In my opinion Marnie Squire was
not at fault in any way, and to conclude
that the plate in this piano broke
because of any negligence or improper
technique on her part would be
utterly absurd.