Escapement: The Part That Makes a Clock Tick
Every tick you hear from a mechanical clock is the escapement at work, letting the gears slip forward one tooth at a time.
The problem an escapement solves
A mechanical clock is powered by something that wants to run down as fast as it can: a falling weight or an unwinding spring. Left alone, the weight would drop to the floor in seconds and spin the gears in a blur.
The escapement is the gatekeeper. It locks the gear train, then lets it advance by a single tooth, then locks it again, over and over. Each release is timed by something that swings back and forth at a steady rate, such as a foliot, a pendulum or a balance wheel.
It also works in the other direction. Each time a tooth slips past, it gives the swinging part a small push, so the oscillator never runs out of energy. The tick you hear is the sound of a tooth landing on the escapement.
See also: a guide to the parts of a clock on OnlineClock.app.
Verge and foliot: the first mechanical clocks
The escapement that made the mechanical clock possible was the verge. Britannica says it was probably invented in 13th-century Europe. NIST notes that large weight-driven clocks using a verge-and-foliot escapement appeared in the towers of several big Italian cities in the first half of the 14th century.
The design has three main parts. A crown wheel, shaped like a crown with teeth around its rim, is driven by the weight. A vertical rod called the verge carries two small flaps, the pallets, set at an angle so that one catches the wheel's teeth on one side and the other catches them on the opposite side.
At the top of the verge sits the foliot, a horizontal bar with small weights at each end. As the crown wheel pushes one pallet aside, the foliot swings until the other pallet catches a tooth and sends it back. Moving the weights outward slows the swing and the clock; moving them in speeds it up.
The University of Bristol calls the verge and foliot the dominant mechanism from the late 13th to the late 17th century, but it had a serious weakness. The foliot has no natural rhythm of its own, so NIST explains that its speed depended heavily on the driving force and on friction. That made it hard to regulate.
The pendulum changes everything
A pendulum, unlike a foliot, does have a natural period. Christiaan Huygens dated his working pendulum clock to 25 December 1656, and he was granted a patent in June 1657. According to NIST, his first pendulum clock erred by less than a minute a day, and later refinements brought that under 10 seconds.
Early pendulum clocks were paired with the old verge, but the partnership did not last long. A verge makes the pendulum swing in a wide arc, and Huygens found that swings wider than a degree or two take slightly longer than narrow ones. He fitted curved cheeks at the top of the pendulum to correct for it.
Large public clocks soon followed. The University of Bristol notes that the first turret clocks with pendulums appeared within a few years of the first domestic pendulum clocks of the late 1650s.
The anchor escapement
The better answer was a new escapement. The anchor escapement was developed in England during the 1660s and 1670s, and NIST records that William Clement began building clocks with it in London in 1671. Who first came up with it is less clear: Britannica and the University of Bristol both describe its development without naming an inventor.
Its pallets are shaped like an upside-down ship's anchor and sit in the same plane as a flat escape wheel. Because it needs only a small swing, the pendulum stays in the narrow arc where its timing is steadiest, and Huygens's cheeks are no longer needed.
The anchor quickly became standard in new tower clocks, as the University of Bristol's study of parish clocks describes, and in the tall longcase clocks we now call grandfather clocks.
It is also called the recoil escapement. As the pendulum finishes each swing, the sloping pallet pushes the escape wheel slightly backward. On a clock with a seconds hand, that can show up as a small backward jerk after each tick.
See also: longcase or grandfather clocks on OnlineClock.app.
Graham's deadbeat: removing the recoil
Recoil means the pendulum is fighting the gears for part of every swing. George Graham, a London clockmaker who partnered Thomas Tompion late in Tompion's life, is said to have modified the anchor to get rid of it around 1715. The result is called the deadbeat or Graham escapement.
The trick is in the shape of the pallets. Every point on a pallet's locking face sits the same distance from the pallets' pivot, so when a tooth lands there the pallet can keep sliding without pushing the wheel backward. The second hand stops dead instead of bouncing.
A separate sloped impulse face at the tip of each pallet gives the pendulum its push. Graham declined to patent his inventions, and Princeton's history of him calls the deadbeat the escapement of choice in almost all finer pendulum clocks since.
How much better did clocks get?
NIST's history of timekeeping gives a sense of the progress. Huygens's pendulum brought errors down to under a minute a day, and later under 10 seconds. Portable watches of the late 17th century, fitted with Huygens's balance wheel and spring, still drifted by about 10 minutes a day.
By 1721, NIST says, Graham had brought pendulum clocks to about one second a day, mainly by correcting for the way heat and cold change a pendulum's length. By 1761, John Harrison's marine timekeeper kept time on a rolling ship to about a fifth of a second a day, and in 1889 Siegmund Riefler's precision clock reached about a hundredth of a second a day.
The idea that kept improving clocks
Later escapement design was guided by a principle Britannica calls detachment: give the oscillator its energy, then keep the escapement as separate from it as possible so it can swing freely. Edmund Denison's gravity escapement of 1854, which still runs the Great Clock at Westminster, is a famous step in that direction.
Seen that way, the history of the escapement is a long effort to let the pendulum or balance swing freely. The less the gears disturb it, the more the clock keeps the oscillator's own steady rhythm.
Questions people ask
What does an escapement do in a clock?
It lets the gear train move forward one tooth at a time, timed by a pendulum or balance, and gives that oscillator a small push with each release to keep it swinging.
What makes a clock tick?
The tick is the sound of an escape wheel tooth striking a pallet of the escapement as the gear train is released and locked again.
What is the difference between an anchor and a deadbeat escapement?
An anchor pushes the escape wheel slightly backward each beat, called recoil. Graham's deadbeat reshapes the pallets so there is no recoil, which improves accuracy.
Who invented the anchor escapement?
It is disputed. It was developed in England in the 1660s and 1670s, and William Clement was building clocks with it in London by 1671.