An automatic or self-winding watch is a mechanical watch, whose mainspring is wound automatically by the natural motion of the wearer's arm, to make it unnecessary to manually wind the watch. Most mechanical watches sold today are self-winding.
How it works
To accomplish this, 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 a ratcheted 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 counter clockwise 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. Usually 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.
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 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 originally 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, it's 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.
This slipping mainspring device is often referred to misleadingly in watch company terminology as an 'unbreakable mainspring'.
The watch winder
For people who do not wear their automatic watch every day, watch winders are available to store automatic watches and keep them wound. This is particularly advantageous if the watch has complex or perpetual calendars or moon phases. A watch winder is a device that can hold one or more watches and moves them in circular patterns to approximate the human motion that otherwise keeps the self-winding mechanism working. A mechanical watch should be kept wound and running as much as possible to prevent its lubricants from congealing over time, which diminishes accuracy. A full service (which involves disassembly, cleaning and re-lubrication) should be performed at least every five years to keep the movement as accurate as possible.
Automatic quartz or kinetic movement
More recently, electronic quartz watches that are powered by arm movement have been developed. A weighted rotor turns a tiny electrical generator, charging a rechargeable battery or capacitor which powers the quartz movement. This automatic quartz arrangement provides the accuracy of quartz without the need to replace the battery or capacitor until it reaches the end of its life, which may be decades.
The display case back
On some automatic watches the rotor and its action are visible through a transparent case back, called a display back or exhibition back. In these watches, the rotors are often engraved or decorated in some way. Rolex is one of the few automatic watch manufacturers that traditionally eschews the transparent case back, mainly for enhanced water resistance.
The Swiss watchmaker Abraham-Louis Perrelet invented a self-winding mechanism in 1770 for pocket watches. It worked on the same principle as a modern pedometer, and was designed to wind as the owner walked, using an oscillating weight inside the large watch that moved up and down. The Geneva Society of Arts reported in 1776 that fifteen minutes walking was necessary to wind the watch sufficiently for eight days, and the following year reported that it was selling well.
Perrelet sold some of his watches to a contemporary watch making luminary, Abraham-Louis Breguet who improved upon the mechanism in his own version of the design, calling his watches "perpetuelles" the French word for perpetual and possibly the source for Rolex's name for its automatic movements, the "Perpetual".
Self winding mechanisms were more successful in wristwatches because the rotor could operate every time that the owner moved his or her arm. However the first version did not appear until the 20th century. It was invented by a watch repairer from the Isle of Man named John Harwood in 1923, who took out a UK patent with his financial backer, Harry Cutts, on 7 July 1923, and a corresponding Swiss patent on 16 October 1923. The Harwood system used a pivoting weight which swung as the wearer moved, and which in turn wound the mainspring. The ratchet mechanism only wound the mainspring when moving in one direction. The weight didn't rotate a full 360?; spring bumpers limited its swing to about 180?, to encourage a back and forth motion. This early type of self-winding mechanism is now referred to as a 'bumper'.
When fully wound, the watch would run for 12 hours autonomously. It did not have a conventional stem winder, so the hands were moved manually by rotating a bezel around the face of the watch. The watches were first produced with the help of fine Swiss watch manufacturer Fortis and went on sale in 1928 and 30,000 were made until the Harwood Self-Winding Watch Company collapsed in 1931 as a result of the Great Depression. 'Bumper' watches were the first commercially successful automatic watches; they were made by several high grade watch manufacturers during the 1930s and 1940s.
The Rolex Watch Company improved Harwood's design in 1930 and used it as the basis for the Rolex Oyster Perpetual, in which the centrally mounted semi-circular weight could rotate through a full 360? rather than the 300? of the 'bumper' winder. Rolex's version also increased the amount of energy stored in the mainspring, allowing it to run autonomously for up to 35 hours.
Most mechanical watches sold today are automatic. A notable exception is the Omega Speedmaster Professional "Moonwatch", the model first used by NASA astronauts during the Apollo Program, and which remains standard issue on all manned space flights.
Because a manually wound wristwatch does not require the weighted rotor which is necessary for an automatic watch, some extremely fine watch companies, such as Patek Philippe, continue to design manually wound watches, which can achieve a case thickness as low as 1.77 millimetres.