Mainspring

The mainspring is the long, flat ribbon of spring alloy coiled inside the barrel that stores the energy a mechanical watch runs on. Winding coils it more tightly around the central arbor. As it unwinds it turns the barrel, whose teeth drive the first wheel of the going train, and that flow of energy is what everything downstream measures out. It is the only power source in a mechanical watch.

The object itself is a ribbon on the order of a tenth of a millimeter thick and a millimeter or two high, long enough that pulled straight it measures several hundred millimeters, coiled inside a barrel roughly 10 to 13 mm across. All the energy the watch will use passes through it, and it is spent again every day or two.

Torque, and why it falls

A mainspring does not deliver a constant pull. A fully wound spring pulls harder than one near the end of its travel, and that falling torque curve feeds straight through to the balance, which swings a little less as the day goes on. Rate changes with it, which is why makers usually quote accuracy over the first 24 hours from full wind and why watchmakers measure a movement at full wind and again a day later.

Materials have improved the picture. Carbon steel springs took a permanent set over time and could snap; modern springs are made from cobalt-based alloys containing nickel and chromium, of which Nivaflex is the best-known trade name, which resist setting and are effectively non-magnetic.

How it sets the power reserve

Reserve is a function of how much spring there is and how fast the movement spends it. A longer, thinner spring in a wider barrel stores more turns, which is the usual route to a 70 hour reserve where a conventional single barrel gives something closer to 38 to 42 hours. Twin barrels and a lower beat rate are the other two levers.

Published figures show the range: the ETA 2824-2 gives 38 hours minimum and 42 typical, the Valjoux 7750 48 and 54, Seiko's 6R35 about 70, and the Powermatic 80 about 80, partly through a patented thinner barrel arbor that fits more spring in the same barrel.

In practice

In a hand-wound watch the outer end of the spring is hooked firmly to the barrel wall, so winding stops dead when the spring is fully coiled. That is your cue to stop, not to give it one more turn.

In an automatic, the outer end carries a slipping bridle that presses against the barrel wall instead of hooking to it. Past full wind it slips, so the rotor can keep turning without harm. This is why wearing the watch cannot overwind it.

Springs tire. A power reserve that has quietly shortened by several hours, or amplitude that will not come up at full wind, often means a set spring or degraded braking grease, and the spring is normally replaced at a full service rather than reused.

Replacement is routine. The spring is a low-value part, supplied coiled in a retaining ring and fitted with a winder, and it comes as part of the job: an indicative US full service on a three-hand automatic is $250 to $500 with an independent before sales tax, per our service costs page. Braking grease matters as much as the spring, because when it dries the bridle grabs instead of slipping.

Terms it gets confused with

The mainspring is not the hairspring: one stores power, the other regulates its release. "Overwound" is the other persistent confusion, since an automatic cannot be overwound and a watch that has stopped while apparently wound has almost always gummed up elsewhere, as our answer on overwinding explains. Nor does a stated reserve tell you how long a watch will run once you take it off; that depends on how full the spring was, the usual reason one stops overnight.