Do you know the Gear Train?

The gear train is responsible for transmitting torque from the barrel to the escapement. It is comprised of wheels and pinions, arranged as a multiplying gear train. In a multiplying gear train, the output rotates faster than the input. This can be thought of in terms of a bicycle; at a high gear, one revolution of the pedals results in many revolutions of the rear wheel.

In other words, a gear train is a mechanical system formed by mounting gears on a frame so the teeth of the gears engage. Gear teeth are designed to ensure the pitch circles of engaging gears roll on each other without slipping, providing a smooth transmission of rotation from one gear to the next.

A byproduct of increasing output speed is a decrease in torque. Continuing the bicycle metaphor, this is why one needs to pedal much harder in the higher gears. So in a watch gear train, the torque at the first wheel is much higher than the torque at the last. In order to minimize this torque loss, special care is taken to lower friction in the gear train. Sapphire jewels are used at the pivots to reduce rotational torque loss. Polished and hardened pinion leaves help reduce torque loss at the meshing of the teeth and leaves.

Usually a gear train consists of four wheels. The center wheel is the first, and is driven by the barrel. It is often located in the center of the movement, and rotates once per hour. The second wheel is next. After the second wheel, we have the third wheel, which rotates once per minute. After the third wheel is the escape wheel.

If the barrel were connected directly to the escapement, a watch would run for only a short period of time. The gear train allows the watch to run for many hours by multiplying the output rotation. The gear train is also responsible for dividing time into useful segments – hours, minutes, and seconds. Beyond its usefulness, a gear train is visually compelling because of its constant movement.