Gear train
The series of wheels reducing the mainspring's speed to readable time
Each wheel steps down the barrel's slow rotation toward the escape wheel's precise frequency.
What it is
The gear train is the series of interlocking wheels and pinions that transfers energy from the barrel to the escapement while simultaneously reducing rotational speed to the correct rates for driving the hands. The center wheel turns once per hour (minute hand); the fourth wheel turns once per minute (seconds hand). The ratios between each wheel pair are calculated to achieve these exact rotations.
History
Gear trains for timekeeping developed with the first mechanical clocks in the 13th century, and the mathematical relationships needed; one revolution per hour, one per 12 hours, one per minute; were worked out empirically over several centuries of clockmaking. By the 17th century the standard configuration was essentially fixed. Modern development focuses almost entirely on manufacturing precision and materials rather than topology: LIGA electroforming (lithographic deep-etch process) produces escape wheels with tolerances measured in micrometres; silicon and titanium reduce mass and inertia in high-frequency movements. The gear train is one of watchmaking's most thoroughly optimised problems; the engineering challenge today is executing a known design to higher and higher tolerances.
How it works
The barrel drives the center wheel, which completes one revolution per hour and carries the cannon pinion that drives the minute hand. The center wheel drives the third wheel (approximately 6–8 revolutions per hour), which drives the fourth wheel; one revolution per minute, which drives the seconds hand directly. The fourth wheel drives the escape wheel via the escapement. Each pair of meshing wheels has a carefully calculated tooth ratio to achieve these rotations. Pivots; the narrow ends of each wheel arbor; run inside jeweled bearings pressed into the plates and bridges; the tolerances at the pivot-jewel interface are measured in microns, and any wear or contamination at these points accelerates timekeeping error.
In the catalog
Related
- Mainspring: The coiled steel strip that stores the watch's energy
- Barrel: The cylindrical container for the mainspring
- Escapement: The mechanism that divides time into equal steps
