'Tired' or 'set' Mainsprings
After decades of use, mainsprings in older timepieces are found to deform slightly and lose some of their force, becoming 'tired' or 'set'. This condition is mostly found in springs in barrels. It causes the running time between windings to decrease. During servicing the mainspring should be checked for 'tiredness' and replaced if necessary. The British Horological Institute suggests these tests:
- In a mainspring barrel, when unwound and relaxed, most of a healthy spring's turns should be pressed flat against the wall of the barrel, with only 1 or 2 turns spiralling across the central space to attach to the arbor. If more than 2 turns are loose in the center, the spring may be 'tired'; with 4 or 5 turns it definitely is 'tired'.
- When removed from the barrel, if the diameter of the relaxed spring lying on a flat surface is less than 2½ times the barrel diameter, it is 'tired'.
Read more about this topic: Mainspring