Mainsprings appeared in the first spring powered clocks, in 15th century Europe. It replaced the weight hanging from a cord wrapped around a pulley, which was the power source used in all previous mechanical clocks. Around 1400 coiled springs began to be used in locks, and many early clockmakers were also locksmiths. Springs were applied to clocks to make them smaller and more portable than previous weight driven clocks, evolving into the first pocket watches by 1600. Many sources erroneously credit the invention of the mainspring to the Nuremberg clockmaker Peter Henlein. However, many references in 15th century sources to portable clocks ‘without weights’, and at least two surviving examples, show that spring driven clocks existed by the early years of that century.
The first mainsprings were made of steel without tempering or hardening processes. They didn’t run very long, and had to be wound twice a day. Henlein was noted for making watches that would run 40 hours between windings. The modern watch mainspring is a long strip of hardened and blued steel, or specialized 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. 24 hours between daily windings with a power-reserve of 12 to 16 hours, in case the owner is late winding the watch. This is the normal standard for hand-wound as well as self-winding watched, used in clocks meant to be wound weekly, provide power for at least 192 hours but use longer mainsprings and bigger barrels. Lock 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Since 1945, carbon steel alloys have been increasingly superseded by newer special alloys (iron, nickel and chromium with the addition of cobalt, molybdenum, or beryllium), and also by cold-rolled alloys (‘structural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
he mainspring is coiled around an axle called the arbor, with the inner end hooked to it. In many clocks, the outer end is attached to a stationary post. The spring is wound up by turning the arbor, and after winding its force turns the arbor the other way to run the clock. The disadvantage of this open spring arrangement is that while the mainspring is being wound, its drive force is removed from the clock movement, so the clock may stop. This type is often used on alarm clocks, music boxes and kitchen timers where it doesn’t matter if the mechanism stops while winding. The winding mechanism always has a ratchet attached, with a pawl (called by clockmakers the click) to prevent the spring from unwinding.
In the form used in modern watches, called the going barrel, the mainspring is coiled around an arbor and enclosed inside a cylindrical box called the barrel which is free to turn. The spring is attached to the arbor at its inner end, and to the barrel at its outer end. The attachments are small hooks or tabs, which the spring is hooked to by square holes in its ends, so it can be easily replaced.
The mainspring is wound by turning the arbor, but drives the watch movement by the barrel; this arrangement allows the spring to continue powering the watch while it is being wound. Winding the watch turns the arbor, which tightens the mainspring, wrapping it closer around the arbor. The arbor has a ratchet attached to it, with a click to prevent the spring from turning the arbor backward and unwinding. After winding, the arbor is stationary and the pull of the mainspring turns the barrel, which has a ring of gear teeth around it. This meshes with one of the clocks gears, usually the center wheel pinion and drives the wheel train. The barrel usually rotates once every 8 hours, so the common 40-hour spring requires 5 turns to unwind completely.