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The mainspring in a mechanical watch is the spring that stores energy of winding to operate the watch between windings. In the USA it is sometimes called the motor spring.

The mainspring is a long strip of hardened and blued steel, or specialised steel alloy, 200–300 millimeters long and 0.05–0.2 millimeters thick. The mainspring is coiled inside the barrel and 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 for self-winding watches. 8-Day movements provide power-reserve for at least 192 hours use in accordance different sizes of mainspring and barrels.

Since 1945, carbon-steel alloys have been increasingly superseded by newer special alloys ( iron-nickel-chrome with the addition of cobalt- molybdenum- beryllium), and also by cold-rolled alloys (structural hardening). As a rule, today's mainsprings are stainless and have a high elastic limit. They are but slightly subject to permanent bending, and there is scarcely any risk of their breaking. Some of them are also practically non-magnetic.

Outside of the barrel, mainsprings can normally have three distinct shapes:

• Spiral coiled: i.e. coiled in the same direction throughout, viz. that of a spring inside the barrel
• Semi-reverse: The outer end of the spring is coiled in the reverse direction to form an angle less than 360 degrees.
• Reverse (resilient): the outer end of the spring is coiled in the reverse direction to form an angle exceeding 360 degrees.

• G.A. Berner 4-language Glossary, 1988 re-edition, courtesy FH, Federation of the Swiss Watch Industry, Bienne, Switzerland

• Federation of the Swiss Watch Industry Glossary

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