The Escapement

A mechanical clock or watch is driven by a falling weight or a wound spring, and timed by an oscillator with a rhythm of its own: a pendulum, or a balance wheel and hairspring. Between the two sits the escapement. Each beat it lets the wheels move on one small step and gives the oscillator a little energy back. The main exhibit is a watch movement beating 28,800 times an hour, with two older clocks beside it.

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Chapters · 13

01All at Once
02Verge and Foliot
03A Rhythm of Its Own
04The Anchor
05The Lever in a Watch
06One Beat in Five Steps
07Left Alone
08The Tick
09Wound and Running Down
10Taken Apart
11Cut Open
12What It Changed
13The Movement in Hand

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01

All at Once

A wound mainspring pushes on the wheels all the time. With nothing to hold them, they spin faster and faster and the stored energy is soon gone.

Timekeeping needs the opposite: the force let out in many equal small portions, at a steady beat. The escapement is that gate, and the oscillator sets the beat.

Pallet fork removed: the train runs free

02

Verge and Foliot

The first mechanical clocks in Europe appeared around 1300, with the verge escapement: teeth at the top and bottom of a crown wheel push in turn on two flags on an upright rod, the verge, which carries a bar on top, the foliot, with a small weight at each end.

The foliot has no rhythm of its own. How long it takes to swing depends on the drive, on friction and on where its weights sit. Early clocks could be an hour a day out; the best were still off by a quarter of an hour.

A heavier driving weight: the foliot swings faster

03

A Rhythm of Its Own

In a letter of 1602 Galileo wrote that a pendulum’s swing takes nearly the same time whatever its size, set by its length. That holds only for small swings: the wider the swing, the longer the period.

In 1656 Christiaan Huygens built the first pendulum clock, letting the pendulum’s period govern the escapement. The error fell to under a minute a day, and with later improvements under 10 seconds. A pendulum about 0.994 m long takes exactly one second per swing: the seconds pendulum.

From a 2.5° to a 5° swing, the clock loses about 31 s a day

04

The Anchor

The anchor escapement, shaped like a ship’s anchor, appeared around 1670 and is usually credited to the London clockmaker William Clement. With it a pendulum needs to swing only a few degrees, far less than the 80° to 90° it needed with a verge, so its period is steadier.

After each tooth escapes, the pendulum swings on a little, and the pallet pushes the escape wheel back before it moves forward again; hence its other name, the recoil escapement. The long pendulum of the longcase clock, beating seconds, comes from here.

Recoil: the pendulum finishes its arc and pushes the wheel back a little

05

The Lever in a Watch

A watch moves with the wrist, so it can’t use a pendulum. Its oscillator is a balance wheel and a hairspring, a very fine spiral spring; the balance swings to and fro under the spring, at a rhythm the two set together.

Between the balance and the escape wheel is the pallet fork. At one end, two ruby pallet stones catch the escape wheel’s teeth; at the other, a notch takes the impulse pin on the balance. The lever escapement was invented in England around 1754, and nearly every mechanical watch today uses its Swiss form, normally with a 15-tooth escape wheel.

Escape wheel · pallet fork and its two stones · impulse pin · balance and hairspring

06

One Beat in Five Steps

The balance swings back, the impulse pin enters the notch and turns the fork, and the stone slides off the tooth: unlocking. Then the tooth runs across the stone’s impulse face, the slope on the tooth pushes too, and the fork passes the push to the pin, giving the balance energy: impulse.

The tooth leaves the stone and the wheel turns freely a little way: drop. The next tooth lands on the other stone’s locking face and stops: lock. The locking face is cut at a slant, so the tooth’s pressure draws the fork against its banking and holds it there. Then the balance finishes its swing on its own.

Unlock → impulse → drop → lock → free swing · slowed down

07

Left Alone

Fully wound, the balance swings about 300° from the middle to each end, and the impulse pin touches the fork only within about 50° around the middle. For the rest of the way the balance touches nothing.

That is why the lever is called a detached escapement: most of the time the balance answers only to its hairspring, and the friction and the changing force in the wheels hardly reach it.

In contact for 8% of the swing’s angle and 5% of its time

08

The Tick

This movement beats 28,800 times an hour: 8 beats a second, the balance swinging back and forth 4 times a second (4 Hz). The escape wheel has 15 teeth, each giving two beats, so 30 beats a turn and 16 turns a minute.

Each beat has a few small knocks: the pin meeting the notch, the tooth landing on the locking face, the fork reaching its banking. The tick you hear is mostly the tooth landing and locking.

8 beats a second · the escape wheel turns 16 times a minute

09

Wound and Running Down

The tighter the mainspring, the more energy each impulse gives and the further the balance swings: typically 270° to 310° fully wound, clearly less as it runs down.

The beat hardly changes: the balance’s period is set by the hairspring and the balance itself. A good movement keeps within about ±10 seconds a day; COSC chronometer certification asks for a mean daily rate between −4 and +6 seconds.

The swing follows the spring; the beat stays

10

Taken Apart

A movement comes apart into the mainspring barrel, the gear train, the escape wheel, the pallet fork, the balance and hairspring, and the main plate and bridges that hold them from below and above.

Every arbor runs in jewel bearings at both ends. The jewels are synthetic ruby, made in quantity by flame fusion since 1902: hard and smooth, they hardly wear.

11

Cut Open

Cut through the centres of the escape wheel, the fork and the balance: every arbor runs in a jewel at each end, the main plate below and the bridges above. The balance staff is the longest, its top jewel held by a shock spring.

Section: escape wheel · pallet fork · balance staff and jewels

12

What It Changed

Once clocks kept time to a few seconds a day, minute hands became usual on clock faces, from about 1690, and second hands followed.

At sea, an accurate clock could find longitude: John Harrison’s H4, completed in 1759, showed it could be done, and later marine chronometers mostly used the detent escapement. Factories worked by the clock, and railways needed one time: the Great Western Railway took London time in 1840, the Railway Clearing House recommended Greenwich time to all lines in 1847, and on 18 November 1883 the railways of the United States and Canada moved to standard time zones.

c. 1300 verge · 1656 pendulum clock · c. 1670 anchor · c. 1754 lever

13

The Movement in Hand

The mainspring sets the swing; the beat stays at 8 a second. Slowed down, one beat stretches out, with the working faces marked on the stones and the pin. The sound is off to begin with; on, it ticks at each lock.

Try thisAs the mainspring runs down, the balance swings less far. Does the watch slow down?

What the model leaves out

The movement is a generic 28,800 vph Swiss lever movement, not any particular calibre; the tooth counts and the shapes of the bridges are the model’s own. The escapement follows textbook proportions: the stones span 2½ teeth, a tooth lands 1.5° deep in lock and the fork runs 0.5° on to its banking, the locking faces are drawn at 12°, and the balance’s lift angle is 50°. Where the escape wheel stands is computed from the teeth and stones touching, not animated by hand. The balance swings as a free oscillator; the impulse’s tiny effect on the period isn’t drawn, and the link between mainspring torque and swing is an illustrative curve. The two clocks are scaled-down models: a seconds pendulum is about a metre long, drawn shorter here but still swinging at two seconds a cycle; the proportions of the crown wheel, the anchor and the foliot are illustrative. The guard pin and safety roller, the inside of the shock setting and the automatic winding are not drawn. Slowed down, the stretch from unlocking to lock is slowed down further still.

Some common misreadings

The escapement doesn’t set the rate: the oscillator does, and the escapement only lets the wheels on and tops up its energy. A pendulum is isochronous only for small swings; wider swings take longer (circular error), which is why Huygens hung his pendulum between cycloidal cheeks. A detached escapement isn’t one that never touches, but one that touches for only a small part of the swing. Who invented the anchor is disputed: it is usually credited to William Clement and sometimes to Robert Hooke, and the oldest surviving anchor clock dates from about 1670. Today’s hairsprings are mostly alloys that resist temperature and magnetism, some of silicon; the blued steel here is the older practice.