In horology

Horology

Horology is the art or science of measuring time. Clocks, watches, clockwork, sundials, clepsydras, timers, time recorders and marine chronometers are all examples of instruments used to measure time.People interested in horology are called horologists...

, the recoil or anchor escapement is a type of escapement

Escapement

In mechanical watches and clocks, an escapement is a device that transfers energy to the timekeeping element and enables counting the number of oscillations of the timekeeping element...

 used in pendulum clock

Pendulum clock

A pendulum clock is a clock that uses a pendulum, a swinging weight, as its timekeeping element. The advantage of a pendulum for timekeeping is that it is a resonant device; it swings back and forth in a precise time interval dependent on its length, and resists swinging at other rates...

s. An escapement

Escapement

In mechanical watches and clocks, an escapement is a device that transfers energy to the timekeeping element and enables counting the number of oscillations of the timekeeping element...

 is the mechanism in a mechanical clock

Clock

A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...

 that maintains the swing of the pendulum

Pendulum

A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...

 and allows the clock's wheels to advance a fixed amount with each swing, moving the hands forward. The anchor escapement was probably invented by British scientist Robert Hooke

Robert Hooke

Robert Hooke FRS was an English natural philosopher, architect and polymath.His adult life comprised three distinct periods: as a scientific inquirer lacking money; achieving great wealth and standing through his reputation for hard work and scrupulous honesty following the great fire of 1666, but...

 around 1657, although some references credit clockmaker William Clement who popularized the anchor in his invention of the longcase or grandfather clock around 1680, and disputed credit for the escapement with Hooke. Joseph Knibb

Joseph Knibb

Joseph Knibb was an English clockmaker.-Biography:He was born as the fifth son of Thomas Knibb, yeoman of Claydon, in 1640. He was cousin to Samuel Knibb, clockmaker, to whom he was apprenticed in 1655. After serving his seven years he moved to Oxford in 1662, the year Samuel moved to London.In...

 probably built the first working anchor clock at Wadham College, Oxford

Oxford

The city of Oxford is the county town of Oxfordshire, England. The city, made prominent by its medieval university, has a population of just under 165,000, with 153,900 living within the district boundary. It lies about 50 miles north-west of London. The rivers Cherwell and Thames run through...

, around 1670. The anchor became the escapement used in almost all pendulum clocks. A more accurate variation called the deadbeat escapement was introduced by British clockmaker Richard Towneley

Richard Towneley

Richard Towneley was an English mathematician and astronomer from Towneley near Burnley, Lancashire. He was one of a group of seventeenth century astronomers in the north of England, which included Jeremiah Horrocks, William Crabtree and William Gascoigne, the pioneer astronomers who laid the...

 around 1675. This gradually superseded the anchor and is used in virtually all modern pendulum clocks.

How it works

The anchor escapement consists of two parts; the escape wheel, which is a vertical wheel with pointed teeth on it rather like saw

Saw

A saw is a tool that uses a hard blade or wire with an abrasive edge to cut through softer materials. The cutting edge of a saw is either a serrated blade or an abrasive...

 teeth, and the anchor, shaped vaguely like a ship's anchor, which swings back and forth on a pivot just above the escape wheel. On the two arms of the anchor are angled flat faces which the teeth of the escape wheel push against, called pallets. The central shaft of the anchor is attached to the pendulum

Pendulum

A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...

, so the anchor swings back and forth,
with the pallets alternately catching and releasing an escape wheel tooth on each side.

Each time one pallet moves away from the escape wheel, releasing a tooth, the wheel turns and a tooth on the other side catches on the other pallet, which is moving toward the wheel. The momentum of the pendulum continues to move the second pallet toward the wheel, pushing the escape wheel backwards for a distance, until the pendulum reverses direction and the pallet begins to move away from the wheel, with the tooth sliding along its surface, pushing it. Then the tooth slides off the end of the pallet, beginning the cycle again.

Neither the anchor escapement nor the deadbeat form, below, are self starting. The pendulum must be given a swing to get them going.

Recoil

The backward motion of the escape wheel during part of the cycle, called recoil, is one of the disadvantages of the anchor escapement. It results in a reversal of the entire wheel train

Wheel train (horology)

In horology, a wheel train is the gear train of a mechanical watch or clock. Although the term is used for other types of gear trains, the long history of mechanical timepieces has created a traditional terminology for their gear trains which is not used in other applications of gears.Watch...

 back to the driving weight with each tick of the clock, causing backlash

Backlash (engineering)

In mechanical engineering, backlash, sometimes called lash or play, is clearance between mating components, sometimes described as the amount of lost motion due to clearance or slackness when movement is reversed and contact is re-established...

 in the wheel train, excessive wear to the gear teeth, and inaccuracy. It can also cause the points of the escape wheel teeth to dig into the pallet surface. The teeth are slanted backward, opposite the direction of rotation, and the surface of the pallets is slightly convex, to prevent this.

Another reason the escape wheel teeth are slanted backward is as a safety measure. If the clock is moved without immobilising the pendulum, the uncontrolled swinging of the pendulum can cause the anchor pallets to collide violently with the escape wheel. The slanted teeth ensure that the flat faces of the anchor pallets hit the sides of the teeth first, protecting the delicate points from being broken.

The more modern deadbeat escapement (below) doesn't have recoil. One way to determine whether an antique pendulum clock has an anchor or deadbeat escapement is to observe the second hand. If it moves backward slightly after every tick, showing recoil, the clock has an anchor escapement.

Crutch and fork

The shaft of the anchor, called the crutch ends in a fork which embraces the shaft of the pendulum, giving it transverse impulses. The pendulum rod is hung from a short straight suspension spring attached to a sturdy support directly behind the anchor. The pivot of the anchor is aligned with the bending point of the spring. This arrangement results in a more stable pendulum support than simply suspending the pendulum directly from the anchor.

Design details

The anchor is very tolerant of variations in its geometry, so its shape varied widely. In late 19th century, in Britain, the usual design was a 90° angle between the pallets, which meant locating the anchor pivot a distance of √2 ≈ 1.4 times the escape wheel diameter from the escape wheel pivot. In a grandfather clock, which had a pendulum which swung once per second, the escape wheel often had 30 teeth, which made the escape wheel rotate once per minute so the second hand

Clock face

A clock face is the part of an analog clock that displays the time through the use of a fixed numbered dial or dials and moving hands. In its most basic form, recognized universally throughout the world, the dial is numbered 1–12 indicating the hours in a 12-hour cycle, and a short hour hand...

 could be attached to its shaft. In a 30 tooth escape wheel the pallets span about 7½ teeth. The impulse angle of the pallets, which determined the swing of the pendulum, was 3°-4°.

History

The anchor was the second widely used escapement in Europe, replacing the 400 year old verge escapement

Verge escapement

The verge escapement is the earliest known type of mechanical escapement, the mechanism in a mechanical clock that controls its rate by advancing the gear train at regular intervals or 'ticks'. Its origin is unknown. Verge escapements were used from the 14th century until about 1800 in clocks...

 in pendulum clock

Pendulum clock

A pendulum clock is a clock that uses a pendulum, a swinging weight, as its timekeeping element. The advantage of a pendulum for timekeeping is that it is a resonant device; it swings back and forth in a precise time interval dependent on its length, and resists swinging at other rates...

s. The pendulums in verge escapement clocks had very wide swings of 80° to 100°. In 1673, 17 years after he invented the pendulum clock, Christiaan Huygens published his mathematical analysis of pendulums, Horologium Oscillatorium. In it he showed that the wide pendulum swings of verge clocks caused them to be inaccurate, because it made the period of oscillation

Oscillation

Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes...

 of the pendulum vary with unavoidable changes in drive force. The widespread realization that only small pendulum swings were isochronous

Isochronous

Isochronous : From Greek iso, equal + chronos, time. It literally means regularly, or at equal time intervals. In general English language, it refers to something that occurs at a regular interval, of the same duration; as opposed to synchronous which refers to more than one thing happening at the...

 motivated clockmakers to look for an escapement that could deliver small swings.

The chief advantage of the anchor was that by locating the pallets farther from the pivot, the swing of the pendulum was reduced from around 100° in verge clocks to only 4°-6°. In addition to the improved accuracy due to isochronism, this allowed clocks to use longer pendulums, which had a slower 'beat'. Lower air drag meant they needed less power to keep swinging, and caused less wear on the clock's movement. The anchor also allowed the use of a heavier pendulum bob

Bob (physics)

A bob is the weight on the end of a pendulum most commonly, but not exclusively, found in pendulum clocks.- Reason for use :Although a pendulum can theoretically be any shape, any rigid object swinging on a pivot, clock pendulums are usually made of a weight or bob attached to the bottom end of a...

 for a given drive force, making the pendulum more independent of the escapement (higher Q

Q factor

In physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....

), and thus more accurate. These long pendulums required long narrow clock cases. Around 1680 British clockmaker William Clement began selling the first commercial clocks to use the anchor escapement, tall freestanding clocks with 1 meter (39 inch) seconds pendulum

Seconds pendulum

A seconds pendulum is a pendulum whose period is precisely two seconds; one second for a swing in one direction and one second for the return swing, a frequency of 1/2 Hz....

s that came to be called longcase

Longcase clock

A longcase clock, also tall-case clock, floor clock, or grandfather clock, is a tall, freestanding, weight-driven pendulum clock with the pendulum held inside the tower, or waist of the case. Clocks of this style are commonly 1.8–2.4 metres tall...

 or 'grandfather' clocks. The anchor increased the accuracy of clocks so much that around 1680–1690 the use of the minute hand

Clock face

A clock face is the part of an analog clock that displays the time through the use of a fixed numbered dial or dials and moving hands. In its most basic form, recognized universally throughout the world, the dial is numbered 1–12 indicating the hours in a 12-hour cycle, and a short hour hand...

, formerly the exception in clocks, became the rule

The anchor escapement replaced the verge in pendulum clocks within about 50 years, although French clockmakers continued to use verges until about 1800. Many verge clocks were rebuilt with anchors. In the 18th century the more accurate deadbeat form of the escapement replaced the anchor in precision regulators, but the anchor remained the workhorse in home pendulum clocks. During the 19th century the deadbeat form gradually took over in most quality clocks, but the anchor form is still used in a few pendulum clocks today.

Disadvantages

The anchor escapement is reliable and tolerant of large geometrical errors in its construction, but its operation is similar to the old verge escapement

Verge escapement

The verge escapement is the earliest known type of mechanical escapement, the mechanism in a mechanical clock that controls its rate by advancing the gear train at regular intervals or 'ticks'. Its origin is unknown. Verge escapements were used from the 14th century until about 1800 in clocks...

, and retains two of the major disadvantages of the verge:

• It is a frictional escapement; the pendulum is always being pushed by an escape wheel tooth throughout its cycle, and never allowed to swing freely. This makes the clock's rate sensitive to changes in the drive force, a problem called lack of isochronism. Any small changes in the force applied to the pallets, by a change in lubrication or the declining force of the clock's mainspring
  Mainspring
  A mainspring is a spiral torsion spring of metal ribbon that is the power source in mechanical watches and some clocks. Winding the timepiece, by turning a knob or key, stores energy in the mainspring by twisting the spiral tighter. The force of the mainspring then turns the clock's wheels as it...
  
   as it runs down for example, will change the period
  Frequency
  Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
  
   of the pendulum's swing. Anchor clocks driven by a mainspring slowed down slightly as the mainspring unwound, and required a fusee to even out the force of the mainspring to be accurate.
• It is a recoil escapement as mentioned above; the momentum of the pendulum pushes the escape wheel backward during part of the cycle. This causes extra wear to the movement, and the backlash
  Backlash (engineering)
  In mechanical engineering, backlash, sometimes called lash or play, is clearance between mating components, sometimes described as the amount of lost motion due to clearance or slackness when movement is reversed and contact is re-established...
  
   of the gear train applies varying force to the pendulum, causing inaccuracy.

Deadbeat escapement

The above two problems were remedied by a modification to the pallets, resulting in a much better variation of the anchor escapement: the Graham or deadbeat escapement. This is often erroneously credited to English clockmaker George Graham

George Graham (clockmaker)

George Graham was an English clockmaker, inventor, and geophysicist, and a Fellow of the Royal Society.He was born to George Graham in Kirklinton, Cumberland. A Friend like his mentor Thomas Tompion, Graham left Cumberland in 1688 for London to work with Tompion...

 who introduced it around 1715 in his precision regulator clocks. However it was actually invented around 1675 by Richard Towneley

Richard Towneley

Richard Towneley was an English mathematician and astronomer from Towneley near Burnley, Lancashire. He was one of a group of seventeenth century astronomers in the north of England, which included Jeremiah Horrocks, William Crabtree and William Gascoigne, the pioneer astronomers who laid the...

, and first used by Graham's mentor Thomas Tompion

Thomas Tompion

Thomas Tompion was an English clock maker, watchmaker and mechanician who is still regarded to this day as the Father of English Clockmaking. Tompion's work includes some of the most historic and important clocks and watches in the world and can command very high prices whenever outstanding...

 in a clock built for Sir Jonas Moore

Jonas Moore

Sir Jonas Moore, FRS was an English mathematician, surveyor, Ordnance Officer and patron of astronomy. He participated in two of the most ambitious English civil engineering projects of the 17th century: the draining of the Great Level of the Fens and the building of the Mole at Tangier...

, and in the two precision regulators he made for the new Greenwich Observatory in 1676, mentioned in correspondence between Astronomer Royal John Flamsteed

John Flamsteed

Sir John Flamsteed FRS was an English astronomer and the first Astronomer Royal. He catalogued over 3000 stars.- Life :Flamsteed was born in Denby, Derbyshire, England, the only son of Stephen Flamsteed...

 and Towneley

The deadbeat form of the anchor escapement was initially used only in precision clocks, but due to its superior accuracy its use spread during the 19th century to most quality pendulum clocks. Almost all pendulum clocks made today use it. The deadbeat escapement does rely on a quality wheel train to receive constant impulse. Variations in the impulse will affect the pendulum arc and the accuracy of the clock. For this reason it is not always the best choice for cheap mass produced clocks, or for tower clocks with large hands exposed to the outside weather.

How it works

The deadbeat has a second face on the pallets, called the 'locking' or 'dead' face, with a curved surface concentric with the axis on which the anchor rotates. When an escape wheel tooth is resting against one of these faces, its force is directed through the anchor's pivot axis, so it gives no impulse to the pendulum, allowing it to swing freely. When the pallet on the other side releases the escape wheel, a tooth lands on this "dead" face first, and remains resting lightly against it for most of the pendulum's outward swing and return. For this period the escape wheel is "locked" and unable to turn. Near the bottom of the pendulum's swing the tooth slides off the dead face onto the slanted 'impulse' face of the pallet, allowing the escape wheel to turn and give the pendulum a push, before dropping off the pallet. The drag of the escape tooth on the dead face does add a small amount of friction to the pendulum's swing, but it is usually negligible.

In contrast to the backward slant of the anchor escape wheel teeth, the deadbeat escape wheel teeth are radial or slant forward to ensure that the tooth makes contact with the 'dead' face of the pallet, preventing recoil.

The Airy condition

In 1826 British astronomer George Airy proved that a pendulum that is driven by a drive impulse that is symmetrical about its bottom equilibrium position is isochronous

Isochronous

Isochronous : From Greek iso, equal + chronos, time. It literally means regularly, or at equal time intervals. In general English language, it refers to something that occurs at a regular interval, of the same duration; as opposed to synchronous which refers to more than one thing happening at the...

 for different drive forces, ignoring friction, and that the deadbeat escapement approximately satisfies this condition. It would be exactly satisfied if the escape wheel teeth were made to fall exactly on the corner between the two pallet faces, but for the escapement to operate reliably the teeth must be made to fall above the corner, on the 'dead' face.

Comparison of motion in anchor and deadbeat

A major cause of error in clocks is changes in the drive force applied to the escapement, caused by small changes in the friction of the gears or the pallets, or the diminishing force of the mainspring

Mainspring

A mainspring is a spiral torsion spring of metal ribbon that is the power source in mechanical watches and some clocks. Winding the timepiece, by turning a knob or key, stores energy in the mainspring by twisting the spiral tighter. The force of the mainspring then turns the clock's wheels as it...

 as it unwinds. An escapement in which changes in drive force do not affect the rate is called isochronous

Isochronous

Isochronous : From Greek iso, equal + chronos, time. It literally means regularly, or at equal time intervals. In general English language, it refers to something that occurs at a regular interval, of the same duration; as opposed to synchronous which refers to more than one thing happening at the...

. The superior performance of the deadbeat over the anchor is caused by its improved isochronism. This is due to the different ways changes in drive force affect the swing of the pendulum in the two escapements:

• In the anchor escapement, an increase of drive force causes the pendulum to swing back and forth more quickly, but does not increase the pendulum's amplitude
  Amplitude
  Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
  
  , the length of its swing, much. The increased force of the escape wheel tooth on the pallet during the recoil part of the cycle tends to decrease the pendulum's swing, while the force of the tooth during the forward impulse part of the cycle tends to increase the pendulum's swing. These tend to cancel each other out, leaving the swing unchanged. But both these effects decrease the time of swing. In other words, increased force knocks the pendulum back and forth in a fixed arc faster.
• In the deadbeat escapement, increased drive force does not change the period
  Periodic function
  In mathematics, a periodic function is a function that repeats its values in regular intervals or periods. The most important examples are the trigonometric functions, which repeat over intervals of length 2π radians. Periodic functions are used throughout science to describe oscillations,...
  
   of the pendulum much, resulting in better isochronism and better timekeeping, but it does increase the pendulum's swing. Since the escapement doesn't have a recoil period when the tooth's force opposes the direction of the pendulum's motion, increased force causes the pendulum to swing in a wider arc, as well as move faster. The time required to cover the extra distance exactly compensates for the increased speed of the pendulum, leaving the period of swing unchanged.

When the deadbeat was invented, clockmakers initially believed it had inferior isochronism to the anchor, because of the greater effect of changes in force on the pendulum's amplitude.

External links

-dead link on Google Books. Details of construction.