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An escapement is a mechanical linkage in and that gives impulses to the timekeeping element and periodically releases the gear train to move forward, advancing the clock's hands. The impulse action transfers energy to the clock's timekeeping element (usually a pendulum or balance wheel) to replace the energy lost to friction during its cycle and keep the timekeeper oscillating. The escapement is driven by force from a coiled spring or a suspended weight, transmitted through the timepiece's gear train. Each swing of the pendulum or balance wheel releases a tooth of the escapement's escape wheel, allowing the clock's gear train to advance or "escape" by a fixed amount. This regular periodic advancement moves the clock's hands forward at a steady rate. At the same time, the tooth gives the timekeeping element a push, before another tooth catches on the escapement's pallet, returning the escapement to its "locked" state. The sudden stopping of the escapement's tooth is what generates the characteristic "ticking" sound heard in operating mechanical clocks and watches.
The first mechanical escapement, the verge escapement, was invented in medieval Europe during the 13th century and was the crucial innovation that led to the development of the mechanical clock. The design of the escapement has a large effect on a timepiece's accuracy, and improvements in escapement design drove improvements in time measurement during the era of mechanical timekeeping from the 13th through the 19th century.
Escapements are also used in other mechanisms besides timepieces. Manual typewriters used escapements to step the carriage as each letter (or space) was typed.
In China, the Tang dynasty Buddhist monk Yi Xing, along with government official Liang Lingzan, made in 723 (or 725) AD the escapement for the workings of a water-powered armillary sphere and drive wheel, which was the world's first clockwork escapement. Song dynasty horologists Zhang Sixun and Su Song duly applied escapement devices for their astronomical clock towers in the 10th century, where water flowed into a container on a pivot. However, the technology later stagnated and retrogressed. According to historian Derek J. de Solla Price, the Chinese escapement spread west and was the source of Western escapement technology.Derek J. de Solla Price, On the Origin of Clockwork, Perpetual Motion Devices, and the Compass, p.86
According to Ahmad Y. Hassan, a mercury escapement described in a Spanish language document for Alfonso X in 1277 can be traced to earlier Arabic language sources.Ahmad Y. Hassan, Transfer Of Islamic Technology To The West, Part II: Transmission Of Islamic Engineering , History of Science and Technology in Islam. Knowledge of these mercury escapements may have spread through Europe with translations of Arabic and Spanish texts.
However, none of these were true mechanical escapements, since they still depended on the flow of liquid through a hole to measure time. In these designs, a container tipped over each time it filled up, thus advancing the clock's wheels each time an equal quantity of water was measured out. The time between releases depended on the rate of flow, as do all liquid clocks. The rate of flow of a liquid through a hole varies with temperature and viscosity changes and decreases with pressure as the level of liquid in the source container drops. The development of mechanical clocks depended on the invention of an escapement which would allow a clock's movement to be controlled by an oscillating weight, which would stay constant.
Astronomer Robertus Anglicus wrote in 1271 that were trying to invent an escapement, but had not yet been successful. Records in financial transactions for the construction of clocks point to the late 13th century as the most likely date for when tower clock mechanisms transitioned from water clocks to mechanical escapements. Most sources agree that mechanical escapement clocks existed by 1300., p.31
However, the earliest available description of an escapement was not a verge escapement, but a variation called a strob escapement. Described in Richard of Wallingford's 1327 manuscript Tractatus Horologii Astronomici on the clock that he built at the Abbey of St. Albans, this escapement consisted of a pair of escape wheels on the same axle, with alternating radial teeth. The verge rod was suspended between them, with a short crosspiece that rotated first in one direction and then the other as the staggered teeth pushed past. Although no other example is known, it is possible that this was the first clock escapement design.
The verge became the standard escapement used in all other early clocks and watches, and remained the only known escapement for 400 years. Its performance was limited by friction and recoil, but most importantly, the early used in verge escapements, known as the foliot, lacked a balance spring and thus had no natural "beat", severely limiting their timekeeping accuracy.
A great leap in the accuracy of escapements happened after 1657, due to the invention of the pendulum and the addition of the balance spring to the balance wheel, which made the timekeepers in both clocks and watches harmonic oscillators. The resulting improvement in timekeeping accuracy enabled greater focus on the accuracy of the escapement. The next two centuries, the "golden age" of mechanical horology, saw the invention of over 300 escapement designs, although only about ten of these were ever widely used in clocks and watches.
The invention of the crystal oscillator and the quartz clock in the 1920s, which became the most accurate clock by the 1930s, shifted technological research in timekeeping to electronics methods, and escapement design ceased to play a role in advancing timekeeping precision.
In many escapements, the unlocking of the escapement involves sliding motion; for example, in the animation shown above, the pallets of the anchor slide against the escapement wheel teeth as the pendulum swings. The pallets are often made of very hard materials such as polished stone (for example, artificial ruby), but even so, they normally require lubrication. Since lubricating oil degrades over time due to evaporation, dust, and oxidation, periodic re-lubrication is needed. If this is not done, the timepiece may work unreliably or stop altogether, and the escapement components may be subjected to rapid wear. The increased reliability of modern watches is due primarily to the higher-quality oils used for lubrication. Lubricant lifetimes can be greater than five years in a high-quality watch.
Some escapements avoid sliding friction, such as the grasshopper escapement of John Harrison in the 18th century. These designs may avoid the need for lubrication in the escapement (though it does not obviate the requirement for lubrication of other parts of the gear train).
Pendulum-based clocks can achieve outstanding accuracy. Even into the 20th century, pendulum-based clocks were reference timepieces in laboratories.
Escapements play a big part in accuracy as well. The precise point in the pendulum's travel at which impulse is supplied will affect how closely to time the pendulum will swing. Ideally, the impulse should be evenly distributed on either side of the lowest point of the pendulum's swing. This is called "being in beat." This is because pushing a pendulum when it is moving towards mid-swing makes it gain, whereas pushing it while it is moving away from mid-swing makes it lose. If the impulse is evenly distributed then it gives energy to the pendulum without changing the time of its swing.
The pendulum's period depends slightly on the size of the swing. If the amplitude changes from 4° to 3°, the period of the pendulum will decrease by about 0.013 percent, which translates into a gain of about 12 seconds per day. This is caused by the restoring force on the pendulum being circular not linear; thus, the period of the pendulum is only approximately linear in the regime of the small angle approximation. To be time-independent, the path must be cycloidal. To minimize the effect with amplitude, pendulum swings are kept as small as possible.
As a rule, whatever the method of impulse, the action of the escapement should have the smallest effect on the oscillator that can be achieved. This effect, which all escapements have to a larger or smaller degree, is known as the escapement error.
Any escapement with sliding friction will need lubrication, but as this deteriorates the friction will increase, and, perhaps, insufficient power will be transferred to the timing device. If the timing device is a pendulum, the increased frictional forces will decrease the Q factor, increasing the resonance band, and decreasing its precision. For spring-driven clocks, the impulse force applied by the spring changes as the spring is unwound, following Hooke's law. For gravity-driven clocks, the impulse force also increases as the driving weight falls and more chain suspends the weight from the gear train; in practice, however, this effect is only seen in large public clocks, and it can be avoided by a closed-loop chain.
Watches and smaller clocks do not use pendulums as the timing device. Instead, they use a balance spring: a fine spring connected to a metal balance wheel that oscillates (rotates back and forth). Most modern mechanical watches have a working frequency of 3–4 hertz (oscillations per second) or 6–8 beats per second (21,600–28,800 beats per hour; bph). Faster or slower speeds are used in some watches (33,600bph, or 19,800bph). The working frequency depends on the balance spring's stiffness (spring constant); to keep time, the stiffness should not vary with temperature. Consequently, balance springs use sophisticated alloys; in this area, watchmaking is still advancing. As with the pendulum, the escapement must provide a small kick each cycle to keep the balance wheel oscillating. Also, the same lubrication problem occurs over time; the watch will lose accuracy (typically it will speed up) when the escapement lubrication starts to fail.
Pocket watches were the predecessor of modern wristwatches. Pocket watches, being in the pocket, were usually in a vertical orientation. Gravity causes some loss of accuracy as it magnifies over time any lack of symmetry in the weight of the balance. The tourbillon was invented to minimize this: the balance and spring are put in a cage that rotates (typically but not necessarily, once a minute), smoothing gravitational distortions. This very clever and sophisticated clockwork is a prized complication in wristwatches, even though the natural movement of the wearer tends to smooth gravitational influences anyway.
The most accurate commercially produced mechanical clock was the electromechanical Shortt-Synchronome free pendulum clock invented by W. H. Shortt in 1921, which had an uncertainty of about 1 second per year. The most accurate mechanical clock to date is probably the electromechanical Littlemore Clock, built by noted archaeologist E. T. Hall in the 1990s. In Hall's paper, he reports an uncertainty of 3 parts in 109 measured over 100 days (an uncertainty of about 0.02 seconds over that period). Both of these clocks are electromechanical clocks: they use a pendulum as the timekeeping element, but electrical power rather than a mechanical gear train to supply energy to the pendulum.
Escapements are challenging to understand because they are bidirectional devices: energy (impulses) to keep the oscillator going passes through the escapement from the wheel train to the oscillator, but timing signals, the locking and release of the escape wheel, which control how fast the wheel train and clock hands advance, pass in the opposite direction from the oscillator to the wheel train.
How much error the escapement impulses cause depends on the oscillator's resonance. This curve is not infinitely "sharp". It has a small natural frequency range around its resonant frequency called the resonance width . In operation the actual frequency of the oscillator can vary randomly within this range in response to variations in the impulse of the escapement, but outside this frequency range the oscillator does not work at all.
The measure of the possible accuracy of a harmonic oscillator as a timekeeper is a dimensionless parameter called the Q factor, which is equal to the resonant frequency divided by the resonance width
The factor depends on how much friction the oscillator has, how many swings it makes before it runs down when it is swinging freely. The less friction the higher the . The is equal to 2π times the ratio of the stored energy in the pendulum or balance wheel to the energy lost to friction during each cycle, which is equal to the energy added by the escapement impulse each cycle. So the larger the is, the smaller the energy loss, the smaller the impulse that has to be applied each cycle to keep it oscillating, and the smaller the disturbance to the oscillator's natural motion. The of balance wheels is around 300, that of pendulums is 103 - 105, while that of quartz crystals in is 105 - 106. This explains why balance wheels are generally less accurate timekeepers than pendulums, which are less accurate than quartz clocks.
Therefore the goal of escapement design is to apply the impulse in a way that minimizes the change in period with changes in drive force. This is called isochronism. No escapement is completely isochronous, but the less a change in drive force disturbs the oscillator, the more accurate the timepiece can be.
Even if the escapement operation were perfectly isochronous, the pendulum or balance wheel itself inevitably has small inherent departures from isochronism, caused by failure of the restoring force to be exactly proportional to amplitude. In balance wheels this is due to small nonlinearities in the balance spring. In pendulums this is due to circular error, a small increase in the period of swing with amplitude.
The verge was the only escapement used in clocks and watches for 350 years. In spring-driven clocks and watches, it required a fusee to even out the force of the mainspring. It was used in the first pendulum clocks for about 50 years after the pendulum clock was invented in 1656. In a pendulum clock, the crown wheel and staff were oriented horizontally, and the pendulum was hung from the staff. However, the verge is the most inaccurate of the common escapements, and after the pendulum was introduced in the 1650s, the verge began to be replaced by other escapements, being abandoned only by the late 1800s. By this time, the fashion for thin watches had required that the escape wheel be made very small, amplifying the effects of wear, and when a watch of this period is wound up today, it will often be found to run very fast, gaining many hours per day.
The anchor consists of an escape wheel with pointed, backward slanted teeth, and a piece pivoted above it, shaped vaguely like a ship's anchor, which rocks from side to side, linked to the pendulum. The anchor has slanted pallets on the arms which alternately catch on the teeth of the escape wheel, receiving impulses. Operation is mechanically similar to the verge escapement, and it has two of the verge's disadvantages: (1) The pendulum is constantly being pushed by an escape wheel tooth throughout its cycle, and is never allowed to swing freely, which disturbs its isochronism, and (2) it is a recoil escapement; the anchor pushes the escape wheel backward during part of its cycle. This causes backlash, increased wear in the clock's gears, and inaccuracy. These problems were eliminated in the deadbeat escapement, which slowly replaced the anchor in precision clocks.
In the deadbeat, the pallets have a second curved "locking" face on them, concentric about the pivot on which the anchor turns. During the extremities of the pendulum's swing, the escape wheel tooth rests against this locking face, providing no impulse to the pendulum, which prevents recoil. Near the bottom of the pendulum's swing, the tooth slides off the locking face onto the angled "impulse" face, giving the pendulum a push, before the pallet releases the tooth. The deadbeat was first used in precision regulator clocks, but because of its greater accuracy it superseded the anchor in the 19th century. It is used in almost all modern pendulum clocks, except for tower clocks, which often use gravity escapements.
The detent is a detached escapement; it allows the balance wheel to swing undisturbed during most of its cycle, except the brief impulse period, which is only given once per cycle (every other swing). Because the driving escape wheel tooth moves almost parallel to the pallet, the escapement has little friction and does not need oiling. For these reasons among others, the detent was considered the most accurate escapement for balance wheel timepieces. John Arnold was the first to use the detent escapement with an overcoil balance spring (patented 1782), and with this improvement his watches were the first truly accurate pocket timekeepers, keeping time to within 1 or 2 seconds per day. These were produced from 1783 onwards.
However, the escapement had disadvantages that limited its use in watches: it was fragile and required skilled maintenance; it was not self-starting, so if the watch was jarred in use so the balance wheel stopped, it would not start up again; and it was harder to manufacture in volume. Therefore, the self-starting lever escapement became dominant in watches.
Rather than pallets, the escapement uses a cutaway cylinder on the balance wheel shaft, which the escape teeth enter one by one. Each wedge-shaped tooth impulses the balance wheel by pressure on the cylinder edge as it enters, is held inside the cylinder as it turns, and impulses the wheel again as it leaves out the other side. The wheel usually had 15 teeth and impulsed the balance over an angle of 20° to 40° in each direction. It is a frictional rest escapement, with the teeth in contact with the cylinder over the whole balance wheel cycle, and so was not as accurate as "detached" escapements like the lever, and the high friction forces caused excessive wear and necessitated more frequent cleaning.
In the duplex, as in the chronometer escapement to which it has similarities, the balance wheel only receives an impulse during one of the two swings in its cycle. , p137-154 The escape wheel has two sets of teeth (hence the name "duplex"); long locking teeth project from the side of the wheel, and short impulse teeth stick up axially from the top. The cycle starts with a locking tooth resting against the ruby disk. As the balance wheel swings counterclockwise through its center position, the notch in the ruby disk releases the tooth. As the escape wheel turns, the pallet is in just the right position to receive a push from an impulse tooth. Then the next locking tooth drops onto the ruby roller and stays there while the balance wheel completes its cycle and swings back clockwise, and the process repeats. During the clockwise swing, the impulse tooth falls momentarily into the ruby roller notch again but is not released.
The duplex is technically a frictional rest escapement; the tooth resting against the roller adds some friction to the balance wheel during its swing but this is very minimal. As in the chronometer, there is little sliding friction during impulse since pallet and impulse tooth are moving almost parallel, so little lubrication is needed. However, it lost favor to the lever escapement; its tight tolerances and sensitivity to shock made duplex watches unsuitable for active people. Like the chronometer, it is not self-starting and is vulnerable to "setting"—if a sudden jar stops the balance during its clockwise swing, it cannot restart.
The design was developed steadily from the middle of the 18th century to the middle of the 19th century. It eventually became the escapement of choice for , because their wheel trains are subjected to large variations in drive force caused by the large exterior hands, with their varying wind, snow, and ice loads. Since in a gravity escapement, the drive force from the wheel train does not itself impel the pendulum but merely resets the weights that provide the impulse, the escapement is not affected by variations in drive force.
The "double three-legged gravity escapement" shown here is a form of escapement first devised by a barrister named Bloxam and later improved by Lord Grimthorpe. It is the standard for all accurate tower clocks. In the animation, the two "gravity arms" are coloured blue and red. The two three-legged escape wheels are also coloured blue and red. They work in two parallel planes so that the blue wheel only impacts the locking block on the blue arm and the red wheel only impacts the red arm. In a real escapement, these impacts give rise to loud audible "ticks" (indicated in the animation by the appearance of an asterisk (*) beside the locking blocks). The three black lifting pins are key to the operation of the escapement. They cause the weighted gravity arms to be raised by an amount indicated by the pair of parallel lines on each side of the escapement. This gain in potential energy is the energy given to the pendulum on each cycle. For the Trinity College Cambridge Clock, a mass of around 50 grams is lifted through 3 mm each 1.5 seconds, which works out to 1 mW of power. The driving power from the falling weight is about 12 mW, so there is a substantial excess of power used to drive the escapement. Much of this energy is dissipated in the acceleration and deceleration of the frictional "fly" attached to the escape wheels. The great clock in Elizabeth Tower at Westminster that rings London's Big Ben uses a double three-legged gravity escapement.
It could be regarded as having its distant origins in the escapement invented by Robert Robin circa 1792, which gives a single impulse in one direction. Yet with the locking achieved by passive lever pallets,Charles Gros 'Echappements' 1914 P.174 the design of the coaxial escapement is more akin to the Fasoldt escapement (another Robin variant), which was invented and patented by the American Charles Fasoldt in 1859.'English and American watches' George Daniels Published 1967Chamberlain 'It's About Time' Pages 428-429, also P.93 which shows a diagrammatic view of the escapement. Chamberlain 1978 Reprint Gros Echappements 1914 P.184 Fig.213 Both Robin and Fasoldt escapements give impulse in one direction only.
The Fasoldt escapement has a lever with unequal drops; this engages with two escape wheels of differing diameters. The smaller impulse wheel acts on the single pallet at the end of the lever, whilst the pointed lever pallets lock on the larger wheel. The balance engages with and is impelled by the lever through a roller pin and lever fork. The lever "anchor" pallet locks the larger wheel; when this is unlocked, a pallet on the end of the lever is given an impulse by the smaller wheel through the lever fork. The return stroke is "dead", with the anchor pallets serving only to lock and unlock, impulse being given in one direction through the single lever pallet. As with the duplex, the locking wheel is larger in order to reduce pressure and thus friction.
Daniels' coaxial escapement, however, achieves a double impulse with passive lever pallets serving only to lock and unlock the larger wheel. On one side, impulse is given by means of the smaller wheel acting on the lever pallet through the roller and impulse pin. On the return, the lever again unlocks the larger wheel, which gives an impulse directly onto an impulse roller on the balance staff.
The main advantage is that this enables both impulses to occur on or around the centre line, with disengaging friction in both directions. This mode of impulse is in theory superior to the lever escapement, which has engaging friction on the entry pallet. For long, this was recognized as a disturbing influence on the isochronism of the balance.
Purchasers no longer buy mechanical watches primarily for their accuracy, so manufacturers had little interest in investing in the tooling required to manufacture coaxial escapements; although finally, Omega adopted it in 1990.
Based on patents initially submitted by Rolex on behalf of inventor Nicolas Déhon, the constant escapement was developed by Girard-Perregaux as working prototypes in 2008 (Déhon was then head of Girard-Perregaux R&D department) and in watches by 2013.
The key component of this escapement is a silicon buckled-blade which stores elastic energy. This blade is flexed to a point close to its unstable state and released with a snap each swing of the balance wheel to give the wheel an impulse, after which it is cocked again by the wheel train. The advantage claimed is that since the blade imparts the same amount of energy to the wheel each release, the balance wheel is isolated from variations in impulse force due to the wheel train and mainspring which cause inaccuracies in conventional escapements.
Parmigiani Fleurier with its Genequand escapement and Ulysse Nardin with its Ulysse Anchor escapement have taken advantage of the properties of silicon flat springs. The independent watchmaker, De Bethune, has developed a concept where a magnet makes a resonator vibrate at high frequency, replacing the traditional balance spring.Monochrome-watches, "The evolution of the escapement and recent innovations", February 2016
This type of clock was widely used as a master clock in large buildings to control numerous slave clocks. Most telephone exchanges used such a clock to control timed events such as were needed to control the setup and charging of telephone calls by issuing pulses of varying durations periodically, such as every second.
Since it is the slave pendulum that releases the gravity lever, this synchronization is vital to the functioning of the clock. The synchronizing mechanism used a small spring attached to the shaft of the slave pendulum and an electromagnetic armature that would catch the spring if the slave pendulum was running slightly late, thus shortening the period of the slave pendulum for one swing. The slave pendulum was adjusted to run slightly slow, such that on approximately every other synchronization pulse the spring would be caught by the armature. (requires Adobe Shockwave Player to display animated content)
This form of clock became a standard for use in observatories (roughly 100 such clocks were manufactured) and was the first clock capable of detecting small variations in the speed of Earth's rotation.
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