How a Mechanical Watch Works

A mechanical watch works by passing the force of its mainspring through a train of wheels to the balance. The wheels transmit that force, while the escape wheel, pallet fork and balance regulate how the mechanism runs.

To see how these parts work together, consider a simple Alpina pocket-watch movement dating approximately to the 1930s. It is wound manually and displays hours, minutes and small seconds at 6 o’clock. Its straightforward layout makes the path from mainspring to balance easy to follow.

The mainspring and barrel

The mainspring is housed inside the barrel and provides the movement’s driving force. Winding the watch puts the spring under tension. That force then passes from the barrel through the wheels of the movement towards the escapement and balance.

How the wheel train carries power

Between the barrel and the regulating components sits a sequence of wheels, each with a defined position and role.

The centre wheel occupies the centre of the movement. Its extended arbor reaches through to the dial side, where the minute hand is fitted through a friction coupling. In this layout, the wheel completes one full revolution every 60 minutes.

The intermediate wheel sits between the centre wheel and the seconds wheel, continuing the transmission of force through the train.

The seconds wheel also has an extended arbor, carrying the small seconds hand. It completes one revolution every minute.

The final wheel in the train is the escape wheel. Its teeth are shaped differently from the conventional teeth of the preceding wheels because they interact with the pallet fork.

How the pallet fork controls the escape wheel

The pallet fork has two ruby pallet stones that engage with the teeth of the escape wheel. In the movement examined, two small pins limit the fork’s travel to either side.

As the fork moves in one direction, one pallet stone releases a tooth. The other pallet stone then locks the escape wheel again. When the fork moves in the opposite direction, the roles reverse and another release takes place.

The result is a sequence of controlled releases. The rate of these releases follows the balance’s alternating motion.

The importance of this control becomes clear when the pallet fork is removed. Winding then sends power through the wheel train, and the escape wheel turns uncontrollably as the spring releases its energy. With the fork fitted again, the wheel is restrained and can advance only as the fork releases it.

The balance and hairspring

The balance wheel works with a coiled spring, the hairspring, which allows it to oscillate from one side to the other.

A small ruby impulse jewel on the underside of the balance engages with an opening at the forked end of the pallet fork. As the balance oscillates, this connection moves the fork from side to side, governing the repeated locking and release of the escape wheel.

These alternations also explain the watch’s beat rate. The number of individual ticks and tocks produced in an hour corresponds to the balance’s alternations over that period.

The impulses that sustain the motion

The relationship between the balance and pallet fork works in both directions. While the balance controls the fork’s movement, the fork delivers small impulses to the balance.

A swing provides a useful comparison. Repeated pushes sustain its movement from one side to the other. In the watch, those pushes come from the force of the mainspring, transmitted through the wheel train and delivered by the pallet fork.

As the balance passes through the central part of its swing, the fork acts on its impulse jewel and gives it another small push. The process repeats on each passage, sustaining the oscillation.

The balance therefore keeps moving while the mainspring supplies the driving force. When that supply is exhausted, the repeated impulses cease.