Mainspring - Modern Mainsprings

Modern Mainsprings

A modern watch mainspring is a long strip of hardened and blued steel, or specialised steel alloy, 20-30 cm long and 0.05-0.2 mm thick. The mainspring in the common 1-day movement is calculated to enable the watch to run for 36 to 40 hours, i.e. with a power-reserve for 12 to 16 hours, which is the normal standard for hand-wound as well as self-winding watches. 8-Day movements, used in clocks meant to be wound weekly, provide power for at least 192 hours but use longer mainsprings and bigger barrels. Clock mainsprings are similar to watch springs, only larger.

Since 1945, carbon steel alloys have been increasingly superseded by newer special alloys (iron, nickel and chromium with the addition of cobalt, molybdenum, or beryllium), and also by cold-rolled alloys ('structural hardening'). Known to watchmakers as 'white metal' springs (as opposed to blued carbon steel), these are stainless and have a higher elastic limit. They are less subject to permanent bending (becoming 'tired') and there is scarcely any risk of their breaking. Some of them are also practically non-magnetic.

In their relaxed form, mainsprings are made in three distinct shapes:

• Spiral coiled: These are coiled in the same direction throughout, in a simple spiral.
• Semi-reverse: The outer end of the spring is coiled in the reverse direction for less than one turn (less than 360°).
• Reverse (resilient): the outer end of the spring is coiled in the reverse direction for one or more turns (exceeding 360°).

The semi-reverse and reverse types provide extra force at the end of the running period, when the spring is almost out of energy, in order to keep the timepiece running at a constant rate to the end.