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Self Winding Knowledge

Automatic Watch

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. 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 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 over winding

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.

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 complications, 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 their 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 four years to keep the movement as accurate as possible.

An expensive watch is more accurate, right?

If this is your first time buying an expensive wristwatch, there is one very important fact you need to know in advance. A RM 200 Timex or Casio digital watch will keep time just as well as, and possibly better than, a RM 80,000 solid gold mechanical Omega, Rolex, or other very fine watch.

If that last statement surprised you, read the rest of this section carefully.

All watches tend to gain or lose a few seconds over a period of time. These are small mechanical or electro-mechanical devices that are counting out 86,400 seconds per day. Even if a watch is 99.9% accurate, it will still be off by a minute and a half in only 24 hours! So even a mediocre wristwatch has to be well over 99.9% accurate to even begin to be useful on an ongoing basis.

Reasonable Accuracy Expectations Seconds gain/loss per day Best

by Type of Watch Worst Typical Best Accuracy
Vintage mechanical watch

in good repair +/-60 +/-15 +/-5 99.9826% Modern mechanical watch
non-certified +/-10 +/-5 +/-2 99.9942% Modern mechanical watch
chronometer certified +6/-4 +/-3 +/-1 99.9977% Modern quartz watch
non-certified (normal) +/-2 +/-1 +/-0.1 99.9998% Modern quartz watch
chronometer certified(rare) +/-0.02 +/-0.02 +/-0.0 99.9999%

While some people desire wristwatches with extremely high accuracy over long periods of time, it is seldom for any reason besides personal satisfaction. The few professions that depend on precision time synchronization (such as astronomy, global navigation, train scheduling, and broadcasting) base their operations on high precision time sources, not consumer wristwatches.

Ultimately, if you are living so close to the edge that having your watch off perfect time by less than a minute bothers you or otherwise throws your life into disarray, you probably need less caffeine and a vacation!