Friday, August 24, 2007

The Remarkable Bulova Accutron

The very brief technical superiority of the Accutron watch is, perhaps, one of the best known facts about the design. The Accutron improved immensely upon the early "electronic" watch, which replaced the mainspring with a battery but established rate with a convention balance and balance spring. The Accutron was, in turn, quickly supplanted by "quartz" designs, which established rate by applying battery current to a quartz crystal and, using the resulting vibration as a reference, powered the analog gear train with a stepper motor. In the conventional wisdom, the much higher quartz frequency made the tuning fork of the Accutron obsolete. What is overlooked in this explanation is that the Accutron did something that has, to my knowledge, never been done before or since. It took the bold step of actually powering the movement with its own escapement. It was as if the balance wheel of a conventional, mechanical watch were used to power the gear train. This was a remarkable idea.

On first examination, one of the most startling aspects of the Accutron is how much traditional, high grade watchmaking is involved in its design and execution. It uses an extremely well-made, traditional, machined brass ebauche with a highly jeweled gear train.

The electronics
By contemporary standards, the electronics of the Accutron are simple, even primitive. Almost as if acknowledging the basic mechanical nature of the design, all electronics are neatly isolated from the rest of the movement in a pair of plastic "kidneys" joined by a simple pair of wires. The coils to activate the tuning fork are integrated into the kidneys. One kidney serves largely as a battery compartment, and the other kidney contains a discrete transistor, one resistor, a capacitor, and a few hand-soldered connections. How simple!

The tuning fork
The tuning fork is a relatively traditional piece, measuring 25 millimeters in length (right). On its left arm, it carries a small post, and, attached to the post, a tiny, square-jeweled pawl (inset, 1) and return spring (2).

Having hit upon the, then novel, idea of establishing rate with an electronically vibrated tuning fork, the engineers had next to consider translating that reference into--movement of the movement. How would a frequency standard derived from the minuscule vibrations of a tuning fork actually translate into both timing and powering the hands of a watch?
A traditional escapement might alternately arrest and release the power of the mainspring at relatively consistent rates. As with later quartz watches, a frequency stabilized circuit might stop and start a stepping motor. But there was no mainspring in the Accutron. Stepper motors, and the circuitry to drive them were then unavailable at anything approaching prices suitable for a wristwatch. The not so obvious answer was to turn it all around and go direct. . .

Bulova engineers arrived at the remarkable and bold decision to quite literally power the gear train of the watch with the vibrations of the tuning fork. The vibrating arm of the fork would oscillate an attached pawl back and forth and the pawl would advance a micro-toothed wheel--tooth by tooth. A second pawl anchored to the ebauche, would serve as a ratchet to prevent reverse movement of the drive wheel. It is here that the Accutron finds itself unique among timekeepers. The frequency standard itself is also the motive force of the movement.


To this day, the 40 year old Accutron stands as an interesting and important contribution to horology. The aerie smoothness of the seconds hand (those are micro teeth on the drive wheel) and the audible hum of the tuning fork are unique among wristwatches. The Accutron is a much more than decent piece of work in traditional horological terms. Compared to most contemporary quartz-referenced wristwatches, the caliber 214 is magnificently constructed. Available in a variety of case styles--including the most-favored SpaceView model, which reveals the technology through the dial, the Accutron caliber 214 is a worthy addition to any collection of timepieces.

based on article by Walt Odets, at

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