The mainspring of a watch is the power house of the movement. The time keeping and regulation of the entire watch is dependant on the mainspring delivering consistent and linear power. Without that equal power no amount of adjustment or regulation will be able to correct for positional errors or inaccuracies in rate over time. Mainsprings retain the energy they receive from winding by coiling tightly around the center arbor. It is always trying to unwind to relieve the tension created by winding. As it unwinds, it transfers its energy to the wheels (gears) in the watch's movement via the meshed teeth on the outside edge of the barrel drum. Regulating the release of this energy in a consistent manner is how your watch keeps accurate time.



1) barrel drum
2) mainspring
3) barrel arbor
4) barrel cover

From a fully wound state a watch with a common 40 hour reserve spring requires about 5 turns to completely unwind, so the barrel rotates once every 8 hours. The maximum number of rotations to completely unwind a typical mainspring should be no more than 8 - 10 turns. The length of the mainspring is determined by factors like the radius of the arbor, the inner radius of the barrel, and the thickness of the metal used to fashion the spring. It usually occupies 50% of the space inside the barrel after the arbor's area is accounted for. A larger barrel will have a greater circumference and be able to contain a longer spring, resulting in a greater power reserve. Some manufacturers use 2 or even 3 barrels to extend the length of the reserve. Even matching the radius of the barrel arbor to the thickness of the mainspring are important to the performance of your watch. This ratio is called the k factor. The equation is simply k = r (barrel arbor radius) ÷ e (mainspring thickness), where k's value should work out to between 10 and 14 for a proper match.

The strength of a mainspring is gauged to deliver the correct power and impulse to the balance wheel to provide sufficient amplitude and also to drive any additional functions such as a date or chronograph. A mainspring's strength is directly proportional to its height - i.e. a mainspring twice the height of another will be twice as strong given the same thickness. Strength is also proportional to the cube of its thickness - i.e. a mainspring twice as thick will be 8 times stronger. The length will be determined by the barrel size, which in turn will determine the power reserve, but also its ability to provide force. This ability is inversely proportional to its length - i.e. a spring twice as long will exert half the force.

One of the most difficult tasks in watchmaking is controlling the energy release from the mainspring. It isn't a constant - it will uncoil with more force when fully wound than when it is nearly exhausted. A mainspring is formed into an S shape because a simple concentric spring does not deliver a flat enough power curve. If a concentric spring where to be used it would be impossible to adjust the watch to give a constant rate over time as the spring unwound. Instead the mainspring is tempered into an S shape to provide a compensating force as it unwinds. In this way the S shaped mainspring is able to provide a near flat power curve. The power will only be significantly different at the extreme ends of the mainspring's reserve.

The term isochronism (meaning "same time" in Greek) refers to how evenly a watch can measure time regardless of the state of wind of the mainspring or the position the watch is held. Contrary to what you'd think, as a mainspring loses power a watch can run faster. This occurs because the balance & hairspring assembly is not driven to its full rotational capacity (called it's amplitude in watch vernacular) due to the lack of force. Since the rotational arc is shorter, it can take less time for the balance wheel to oscillate and therefore makes the watch tick faster. There are other many other variables, and a change in amplitude does not always mean a difference in a watch's timekeeping rate.

a mainspring inside its barrel:



a mainspring uncoiled:




Until the 1960's mainsprings usually broke from metal fatigue. Their replacement, either from breaking or wearing out (called a tired spring), was the #1 cause for repair. The material used to make mainsprings has advanced along the same lines as with the hairspring, starting with steel and evolving to better metals that are more resistant to snapping, less susceptible to magnetic influence, and have a longer lifespan. Alloys now are structurally hardened combinations of cobalt, nickel, and chromium that can withstand high amounts of stress while still being ductile and corrosion resistant.

When automatic winding watches became more prevalent a slipping clutch (called a sliding bridle) was added as a buffer between the mainspring and the barrel which the tail end of the mainspring fit into. The inner wall of the barrel has 8 shallow notches etched into it.



Normal tension from the spring's outward push holds the bridle in place. When the spring reaches peak tension the force is greater than the holding power the bridle and it slips which prevents damage to the spring from over-winding. The consecutive notch slipping gives rise to the familiar "tick-tick" feedback we feel when we've fully wound a mechanical watch. Modern watches simply have a bridle welded or riveted to the end of the mainspring, and sometimes don't require the notches in the barrel wall at all.

a bridle welded to the end of the mainspring: