Mainspring - Broken Mainsprings

Broken Mainsprings

Because they are subjected to constant stress cycles, up until the 1960s mainsprings generally broke from metal fatigue long before other parts of the timepiece. They were considered expendable items. This often happened at the end of the winding process, when the spring is wound as tightly as possible around the arbor, with no space between the coils. When manually winding, it is easy to reach this point unexpectedly and put excessive pressure on the spring. Broken mainsprings were the largest cause of watch repairs until the 1960s. Since then, the improvements in spring metallurgy mentioned above have made broken mainsprings rare.

Even if the spring didn't break, too much force during winding caused another problem, called 'knocking' or 'banking'. Since no more slack was left in the spring, the pressure of the last turn of the winding knob put the end of the spring under excessive tension, which was locked in by the last click of the ratchet. So the watch ran with excessive drive force for several hours, until the extra tension in the end of the spring was relieved. This made the balance wheel rotate too far in each direction, causing the impulse pin on the wheel to knock against the back of the fork horns. This caused the watch to gain time, and could break the impulse pin. In older watches this was prevented with 'stopwork'. In modern watches this is prevented by designing the 'click' with some 'recoil' (backlash), to allow the arbor to rotate backward after winding by about two ratchet teeth, enough to remove excess tension.