Watch movements

Written by James Matthews

Watch movements

A movement in watchmaking is the mechanism that measures the passage of time and displays the current time (and possibly other information including date, month and day). Movements may be entirely mechanical, entirely electronic (potentially with no moving parts), or a blend of the two. Most watches intended mainly for timekeeping today have electronic movements, with mechanical hands on the face of the watch indicating the time.

Purely mechanical watches are still popular, although they are most commonly seen among expensive, collectible watches such as Fortis, Omega, Rolex and TAG Heuer. Their superb craftsmanship accounts for much of the attraction of purely mechanical watches. Compared to electronic movements, mechanical watches keep very poor time, often with errors of seconds per day. They are frequently sensitive to position and temperature, they are costly to produce, they require regular maintenance and adjustment, and they are more prone to failure.

Generally speaking, inexpensive and moderately priced timepieces with electronic movements now provide most users with timekeeping more accurate than the most expensive Rolex. However, in recent times there has been less emphasis on time precision as many people now carry multiple devices that will tell them the time such as mobile phones, PDAs and laptops, so finely crafted mechanical watches have remained popular less as time pieces and more because of their aesthetic value as jewelry.

Tuning fork watches (introduced by Bulova in 1960) use a 360 hertz tuning fork to drive a mechanical watch. Since the fork is used in place of a typical balance wheel, these watches naturally hum instead of tick.

The inventor, Max Hetzel, was born in Basel, Switzerland, and joined the Bulova Watch Company of Bienne, Switzerland, in 1948. Hetzel was the first to use an electronic device, a transistor, in a wristwatch. Thus, he developed the first watch that could be qualified as electronic. However, fork movements are actually more “electrical”, like an old electrical wall clock, than electronic. The sweep second hand moves fluidly like that of an old electrical wall clock.

Such watches were also sold by Swiss watch companies under license of Bulova. In 1974, after leaving Bulova, Hetzel developed a different tuning fork drive for Omega Watches. The watch featured a cal. 1220 micromotor, and a tuning fork frequency of 720 hertz.[3] This development was obsolete compared to the newer electronic quartz watch which had become cheaper to produce and even more accurate.

Tuning fork movements are electromechanical. The task of converting electronically pulsed fork vibration into rotary movement is done via two tiny jeweled fingers, called pawls, one of which is connected to one of the tuning fork’s tines. As the fork vibrates, the pawls precisely ratchet a tiny index wheel. This index wheel has over 300 barely visible teeth and spins more than 38 million times per year. The tiny electric coils that drive the tuning fork have 8000 turns of insulated copper wire with a diameter of 0.015 mm and a length of 90 meters. This amazing feat of engineering was prototyped in the 1950s.

Electronic movements have few or no moving parts. Essentially, all modern electronic movements use the piezoelectric effect in a tiny quartz crystal to provide a stable time base for a mostly electronic movement: the crystal forms a quartz oscillator which resonates at a specific and highly stable frequency, and which can be used to accurately pace a timekeeping mechanism. For this reason, electronic watches are often called quartz watches. Most quartz movements are primarily electronic but are geared to drive mechanical hands on the face of the watch in order to provide a traditional analog display of the time, which is still preferred by most consumers.

The first prototypes of electronic quartz watches were made by the CEH research laboratory in Switzerland in 1962. The first quartz watch to enter production was the Seiko 35 SQ Astron, which appeared in 1969. Modern quartz movements are produced in very large quantities, and even the cheapest wristwatches typically have quartz movements.

The best quartz movements are significantly more accurate than the worst, but the difference is much smaller than that found between mechanical movements and quartz movements. Quartz movements, even in their most inexpensive forms, are an order of magnitude more accurate than purely mechanical movements. Whereas mechanical movements can typically be off by several seconds a day, an inexpensive quartz movement in a child’s wristwatch may still be accurate to within 500 milliseconds per day—ten times better than a mechanical movement.

Quartz mechanisms usually have a resonant frequency of 32768 Hz, chosen for ease of use (being 215). Using a simple 15 stage divide-by-two circuit, this is turned into a 1 pulse per second signal responsible for the watch’s keeping of time.

Some electronic quartz watches are able to synchronize themselves with an external time source. These sources include radio time signals directly driven by atomic clocks, time signals from GPS navigation satellites, the German DCF77 signal in Europe, and others. These watches are free-running most of the time, but periodically align themselves with the chosen external time source automatically, typically once a day.

Because these watches are regulated by an external time source of extraordinarily high accuracy, they are never off by more than a small fraction of a second a day (depending on the quality of their quartz movements), as long as they can receive the external time signals that they expect. Additionally, their long-term accuracy is comparable to that of the external time signals they receive, which in most cases (such as GPS signals and special radio transmissions of time based on atomic clocks) is better than one second in three million years. For all practical purposes, then, radio-controlled wristwatches keep near perfect time.

Movements of this type synchronize not only the time of day but also the date, the leap-year status of the current year, and the current state of daylight saving time (on or off). They obtain all of this information from the external signals that they receive. Because of this continual automatic updating, they never require manual setting or resetting.

A disadvantage of radio-controlled movements is that they cannot synchronize if radio reception conditions are poor. Even in this case, however, they will simply run autonomously with the same accuracy as a normal quartz watch until they are next able to synchronize.

This post was written by James Matthews. If you have any questions or comments you may contact him at james@jewelerslounge.com