GEAR MECHANISM USED IN WRIST WATCHES

The underside view of a Gruen automatic watch with a transparent case back, showing an ETA 2824-2. The rotor

and the balance wheel are plainly visible

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.

How it works

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.[1] 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.[2]

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,[3] 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.[4]

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.[5]

A further advantage of this device is that the mainspring can't be broken by overzealous manual

winding.

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 an accumulator, such as 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. This is also called Kinetic movement, which was first introduced by Seiko.

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

hascomplications, like 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. Older mechanical watches should be kept

wound and running as much as possible to prevent its lubricants from congealing over time, which

diminishes accuracy. Modern mechanical watches generally use synthetic oil; whether or not synthetic

oils congeal is a point of contention among watch experts. 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.

History

Perrelet: 1770

Abraham-Louis Perrelet working on a watch

The Swiss watchmaker Abraham-Louis Perrelet invented a self-winding mechanism in 1770 for pocket

watches. It worked on the same principle as a modernpedometer, 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.[6]

[edit]Breguet: 1780

Perrelet sold some of his watches to a contemporary watch making luminary, Abraham-Louis

Breguet around 1780 who improved upon the mechanism in his own version of the design, calling his

watches "perpetuelles" the French word for perpetual.[7][8] They did not work reliably and Breguet

stopped producing them around 1800.[9]

Self-winding mechanisms by August Ritter von Löhr of Austria patented in the 1880's [10]

Bumper' wristwatches: 1923

Self winding mechanisms were more successful in wristwatches because the rotor could operate every

time that the owner moved his or her arm. The first version did not appear until after World War I, when

wristwatches became popular. It was invented by a watch repairer from the Isle of Man named John

Harwood in 1923,[11] 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.[12] 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.[13] This early type of

self-winding mechanism is now referred to as a 'hammer' or 'bumper'.

When fully wound, Harwood's 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 Swiss watch manufacturer Fortis and went

on sale in 1928. 30,000 were made before 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.

Rolex

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.

By the 1960s, automatic winding became standard in quality mechanical watches. Because the

weighted rotor needed in an automatic watch takes up a lot of room in the case, increasing the

thickness of the watch, some high end watch companies, such as Patek Philippe, continue to design

manually wound watches, which can achieve a case thickness as low as 1.77 millimeters.