How A Mechanical Watch Actually Works

A mechanical watch keeps time using energy stored in a spring, released at a controlled rate by an oscillating system. The principle is centuries old and the execution is what distinguishes one watch from another.

The power source

Energy is stored in a coiled mainspring inside a barrel, wound by hand or, in an automatic watch, by a rotor that turns as the wearer moves. The duration the watch runs unworn is its power reserve.

A mainspring delivers more force when fully wound than when nearly unwound, and managing that variation is one of the design problems a movement must solve.

The gear train

A series of wheels transmits energy from the barrel toward the escapement while stepping down the speed, and it is this train that drives the hands at their respective rates.

The escapement

The escapement is the mechanism that both releases energy in measured increments and delivers impulses to keep the oscillator moving. The ticking sound is the escapement operating.

The dominant design in modern watches is the lever escapement, valued for reliability rather than for ultimate precision.

The oscillator

A balance wheel attached to a hairspring swings back and forth at a consistent rate, and that rate is what the watch counts. Its frequency determines how finely time is divided, which is why a higher-frequency movement can display fractions of a second more precisely.

Regulating a watch means adjusting the effective length of the hairspring, which changes the rate.

Why they drift

Rate is affected by position, because gravity acts differently on the balance depending on orientation. It is affected by temperature, by the state of wind, by lubricant condition and by magnetism.

Chronometer certification tests a movement's rate across positions and temperatures against a defined standard, which is why the term means something specific rather than merely accurate.

Servicing

Lubricants degrade and parts wear. A mechanical watch requires periodic servicing, and one run long past its interval wears metal rather than oil — a considerably more expensive problem.