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ISEGORIABenjamin Haire
日本語

A mechanical watch, slowed down

How motion
becomes time.

I think of an automatic watch as a tiny energy-management machine. Your wrist winds a spring; gears carry its power; an oscillator and escapement divide that power into equal, countable beats.

Open the movement ↓

01 / Energy path

The whole movement

Seen from the back, with the cover of the barrel cut away. Select a part, or its label, to read its job.
Interactive illustration of an automatic watch movementA rotor turns a reversing wheel and a ratchet wheel that wind the mainspring in its barrel. The barrel drives the centre, third and fourth wheels and the escape wheel; the pallet fork releases the escape wheel one step per beat of the balance wheel and hairspring under the balance cock.

    Try this: stop the wrist and set the time to ×3600. The reserve falls, the balance swings less far as the spring torque drops, and when the spring is empty everything stops. Move the wrist again and the rotor winds it back up through the reversing wheel. The coils in the barrel show the state: wound tight round the arbor when full, lying against the wall when empty. The same layout is built in three dimensions in the 3D movement lab.

    02 / Going train

    The gear train,
    slowed down

    Each arbor carries a large wheel and a small pinion. A wheel drives the next pinion, so torque is traded for speed at every stage, and every mesh reverses the direction.
    The going train laid out in a lineThe mainspring barrel drives the centre, third and fourth wheels through their pinions, then the escape wheel, whose release is gated by the pallet fork and the balance wheel. Arrows show the direction of rotation alternating at each mesh.

    Tooth counts are typical of a 28 800 vph calibre, not those of a particular one. The ratios are real: run it at ×60 and the fourth wheel turns once a second, as the seconds hand would in a minute.

    Large wheel → small pinion

    80 teeth driving 10 leaves multiplies speed by eight. Four such stages turn one barrel revolution in about 6.7 hours into twelve escape-wheel turns a minute.

    Every mesh reverses direction

    Clockwise becomes anticlockwise at each contact; the gold and blue arrows alternate along the train.

    The escapement is the gate

    The train wants to unwind continuously. The pallet fork lets it advance half a tooth per beat, which is why the escape wheel steps rather than turns.

    03 / The timekeeper

    Where the watch gets its “time”

    A wristwatch uses a balance wheel and hairspring, not a gravity pendulum. The Swiss lever escapement connects it to the train. It is shown here in slow motion, and the moment of contact at the centre of each swing is slowed further still.
    Swiss lever escapement in slow motionThe escape wheel is held by a pallet stone while the balance swings freely. When the impulse jewel on the balance roller enters the fork, it unlocks the wheel; the released tooth pushes the pallet, which pushes the balance, and the other stone catches the next tooth.

    1. 1
      Lock

      The balance swings freely. A pallet stone holds the escape wheel; the fork rests against a banking pin.

    2. 2
      Unlock

      The impulse jewel enters the fork and turns it a little, sliding the stone off the tooth.

    3. 3
      Impulse

      The tooth slides along the stone’s slanted face and pushes the fork, which pushes the balance: a tiny top-up of energy.

    4. 4
      Drop and count

      The wheel jumps half a tooth and the other stone catches the next one. Each of these steps is one beat the train counts.

    04 / Balance + hairspring lab

    Change the oscillator.
    Watch the rate respond.

    Frequency is governed mainly by spring stiffness and balance inertia. The motion below is slowed eight times.
    Beat rate28,800 vphset by inertia + stiffness
    Full oscillation0.250 sout and back
    Instantaneous stateCentre crossingmaximum kinetic energy
    Interactive balance wheel and hairspring oscillatorControls alter inertia, stiffness, and amplitude while the illustration shows the resulting oscillation of a screw balance and its hairspring.inertia carries the wheel through the centre; the spring always pulls it backrestoring torque, always toward centredirection of motion

    Centre crossing: the spring is relaxed and the wheel is moving fastest.

    The model I keep coming back tofrequency ∝ √(spring stiffness ÷ balance inertia)

    A stiffer spring raises the beat rate; a heavier or larger balance lowers it. Amplitude changes travel, but in an ideal oscillator it does not set the basic period. Real watches need careful geometry, adjustment, lubrication, and temperature compensation, none of which I have modelled here.

    The governing idea

    Power is continuous.
    Time is released in steps.

    The part that took me longest to see is that the mainspring always pushes while the escapement keeps the train locked most of the time. Each half-swing unlocks a tooth, receives a tiny impulse, and locks the train again. The watch counts these regularly spaced releases and displays their accumulation as time.