Things That go Crack in the Night I
There are many things which go crack in the middle of the night, agraffes, harps, soundboards, just to name a few. The next couple of installments will look at breaks, not necessarily caused by use, and their repair.
I must be lucky or unlucky depending on your perspective, I have been in the presences of a number of harps which chose that moment to break. One in particular, I won’t ever forget - I had just sat up from looking at the harp (I don’t remember why I was looking) in a spinet when the harp tore below the treble hitch pin field. That will get your attention!
Having dealt with a couple dozen broken or cracked plates, I have found the causes of cracks fall into three basic categories.
1) Casting. Sometimes, especially in the thicker parts of the harp, cracks can appear as the metal cools during it manufacture. I have found a number of small cracks in plate flanges and horns, occasionally these are harmless, but will often fail late in life, often after rebuilding.
2) Structural failures are most common in open face harps. Often in this style there is not enough metal in the bars crossing the pin field to withstand the unique stresses in these pianos. Consequently the struts often break just in front of the tuning pin field. You should always examine this type of harp design carefully for failures in this area.
3) Stresses. If you have pulled out even a few plates, you soon discoverer harps rarely rest squarely on whatever suspension system is present. Many times the dowels or rim liners are not set at the correct height. Other times the nose bolts are suspending the harp or in some cases the nose bolts and nuts bend the harp toward the soundboard. Cast grey iron will flex some but its inflexibility is one of its characteristics which make it good to use in piano harps. These stresses can result in cracked or broken struts. If a strut breaks, you will often see the two parts move away from each other because of internal stresses found in cast iron.
As grey iron cools, after being cast, it develops a grain structure not unlike that found in wood. While not having all the characteristics of wood grain it does represent stress lines within the metal. In the same way that the two halves of a piece of wood partially ripped may move in different directions, cast iron will sometimes move in a similar manner. Because of this characteristic, welding cast iron is difficult at best. Only someone very familiar with cast iron and who has adequate facilities should ever attempt to weld a piano harp. If done incorrectly the welder could chase the crack all around the harp, or the harp could break adjacent to the weld.
When there is insufficient mechanical strength to support the string tension, as is common with open face pinblocks, additional material can be attached to the harp to strengthen the struts. These can be attached mechanically, welding, or by brazing. You will need a machinist to make and fit the plates. There was a recent PTG Journal article detailing such a repair.
For general failures in pin fields and struts, (not do to poor design) I use the Lock & Stitch (tm) system. This system has two parts. The first is the lock which fits across the breaks in a series of over lapping holes drilled as a channel across the break. Smaller locks are used in most strut and tuning pin field failures and mid-sized locks are used in hitch pin field tears.
The stitch is a series of interlocking, inverted thread screws which act to pull the crack together, these are installed the length of the crack.
This is not a difficult repair technique and is used to repair all sorts of very heavy industrial machinery and combustion engines. It can be done with the harp in the piano in some cases, though the jigs are a bit expensive for a one time use. If you have any questions, feel free to give me a call. And about those harps which broke in my presence, all were my own pianos, which were being subjected to experimental rebuilding techniques.