If you think of any object that is under tension, such as a rope or a rubber band, we know that if the material is compromised in any way, that is the most likely reason for failure. A single piano string does not have strands but can weaken to the point of breakage when things such as rust that corrodes the metal on the string. A piano string can break at any time, during tunings, during normal or excessive playing, or even during the day/night when no one is even playing it.
Defects in the string, or built-in problems with the piano, may be responsible for certain kinds of breakage. The pressure bar may have an excessive V-shape where the string passes over/under it; when struck hard with a hammer, this V-shape acts like cutters on pliers and weakens the string until it breaks. This problem was common on early pianos built by one manufacturer but is not found on modern instruments made by other makers.
Two additional factors that can contribute to string breakage are worn or hard hammers, and a piano out of regulation.
Hammers that are worn can get very flat on the crown where the hammer contacts the string. A properly shaped hammer has a sort of tear drop shape that is very symmetrical and the point that contacts the string is relatively small. The hammer hardness should allow for a good rebound off the string, kind of like a super ball…but definitely not hard like a marble or a soft like a cotton ball. When the hammer is flat and worn on the crown, there is more surface area, and more force applied to the string which can cause breakage.
Another contributing factor is a piano out of regulation. When a piano is badly out of regulation, it throws many things out of whack, including how much force it takes to throw the hammer to the strings. You should not have to pound the keys to get a medium loud sound. If you do have to, this is a good sign that there is (what we technicians call) lost-motion, which means that your key is moving downward a bit before the jack ever contacts the hammer to start the hammer’s movement toward the string. The more lost motion, the less power you have, and the more you feel you need to bang the keys to get it to play.
In conclusion, any one, or a combination of these factors can cause string breakage. If you are having an issue of broken strings, please do not hesitate to call us to see if there are any repairs or regulation that needs to be taken care of.
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