An automatic or self-winding watch is a mechanical watch, whose mainspring is wound automatically by the natural motion of the wearer's arm, providing energy to run the watch, to make it unnecessary to manually wind the watch. Most mechanical watches sold today are self-winding.
A self-winding watch movement is similar to a manual movement with the addition of a mechanism powered by an eccentric weight which winds the mainspring.The watch contains a semicircular 'rotor', an eccentric weight that turns on a pivot, within the watch case. The normal movements of the user's arm and wrist cause the rotor to pivot back-and-forth on its staff, which is attached to aratcheted winding mechanism. The motion of the wearer's arm is thereby translated into the circular motion of the rotor that, through a series of reverser and reducing gears, eventually winds the mainspring. Modern self-winding mechanisms have two ratchets and wind the mainspring during both clockwise and counterclockwise rotor motions.
The fully-wound mainspring in a typical watch can store enough energy reserve for roughly two days, allowing automatics to keep running through the night while off the wrist. In many cases automatic watches can also be wound manually by turning the crown, so the watch can be kept running when not worn, and in case the wearer's wrist motions are not sufficient to keep it wound automatically.
Preventing overwinding
A problem that had to be solved with self-winding mechanisms is that they continued working even after the mainspring was fully wound up, putting excessive tension on the mainspring. This could break the mainspring, but even when it didn't, it caused a problem called 'knocking' or 'banking'. The excessive drive force applied to the watch movement gear train made the balance wheel rotate with too much amplitude, that is too far in each direction, causing the impulse pin to hit the back of the pallet fork horns. This made the watch run fast, and could break the impulse pin. To prevent this, a slipping clutch device is used on the mainspring so it cannot be overwound.
The slipping spring or 'bridle'
The 'slipping mainspring' device was patented by Adrien Philippe, founder of Patek Philippe, on June 16, 1863, long before self-winding watches. It was invented to allow simultaneous winding of two mainspring barrels. In an ordinary watch mainspring barrel, the outer end of the spiral mainspring is attached to the inside of the barrel. In the slipping barrel, the mainspring is attached to a circular steel expansion spring, often called the 'bridle', which presses against the inside wall of the barrel, which has serrations or notches to hold it.
As long as the mainspring is less than fully wound, the bridle holds the mainspring by friction to the barrel wall, allowing the mainspring to be wound. When the mainspring reaches full wind, its force is stronger than the bridle spring, and further winding pulls the bridle loose from the notches and it simply slides along the wall, preventing the mainspring from being wound further. The bridle must grip the barrel wall with just the right force to allow the mainspring to wind fully but not overwind. If it grips too loosely, the mainspring will begin to slip before it is fully wound, a defect known as 'mainspring creep' which results in a shortened reserve power time.
A further advantage of this device is that the mainspring can't be broken by overzealous manual winding. This feature is usually advertised by watch companies by putting the words "unbreakable mainspring" on the watch's face or back.
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