Automatic watch

An automatic watch, also known as a self-winding watch or simply an automatic, is a mechanical watch where the natural motion of the wearer provides energy to wind the mainspring, making manual winding unnecessary if worn enough. It is distinguished from a manual watch in that a manual watch must have its mainspring wound by hand at regular intervals.

In a mechanical watch the watch's gears are turned by a spiral spring called a mainspring. In a manual watch, energy is stored in the mainspring by turning a knob, the crown, on the side of the watch. Then the energy from the mainspring powers the watch movement until it runs down, requiring the spring to be wound again.

A self-winding watch movement has a mechanism which winds the mainspring using the natural motions of the wearer's body. The watch contains an oscillating weight that turns on a pivot. The normal movements of the watch in the user's pocket (for a pocketwatch) or on the user's arm (for a wristwatch) cause the rotor to pivot on its staff, which is attached to a ratcheted winding mechanism. The motion of the watch is thereby translated into circular motion of the weight which, through a series of reverser and reducing gears, eventually winds the mainspring. There are many different designs for modern self-winding mechanisms. Some designs allow winding of the watch to take place only while the weight swings in one direction while other, more advanced, mechanisms have two ratchets and wind the mainspring during both clockwise and anti-clockwise weight motions.

The fully wound mainspring in a typical watch can store enough energy reserve for roughly two days, allowing the watch to keep running through the night while stationary. In many cases automatic wristwatches 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.

Self-winding mechanisms continue working even after the mainspring is fully wound up. If a simple mainspring was used, this would put excessive tension on the mainspring. This could break the mainspring, and even if it did not, it can cause a problem called "knocking" or "banking". The excessive drive force applied to the watch movement gear train can make the balance wheel rotate with excessive amplitude, causing the impulse pin to hit the back of the pallet fork horns. This will make 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 mainspring device was patented by Adrien Philippe, one of the founders of Patek Philippe, on 16 June 1863, long before self-winding wristwatches. 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 bridle will begin to slip before the mainspring 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 cannot be broken by excessive manual winding. This feature is often described in watch company advertising as an unbreakable mainspring.