In mechanical watches and clocks, an escapement is a device which converts continuous rotational motion into an oscillating or back and forth motion. The term is also used in fisheries science to refer to that portion of the spawning stock surviving fishing pressures over a spawning cycle.
An escapement drives the timekeeping element, usually a pendulum or balance wheel, in a clock or watch. It is powered by a gear train which is typically turned by a coiled spring or weight. Without the escapement, the system would simply unwind uncontrollably, but the escapement regulates this motion, controlled by the periodic swing of the pendulum or balance wheel. It allows the gears to advance or 'escape' a fixed amount with each swing, moving the timepiece's hands forward at a steady rate. A second function of the escapement is to keep the pendulum or balance wheel moving by giving it small pushes.
Each swing of the pendulum releases the escapement, making it change from a "locked" state to a "drive" state for a short period that ends when the next tooth on the gear hits the locking surface on the escapement. It is this periodic release of energy and rapid stopping that makes a clock "tick;" it is the sound of the gear train suddenly stopping when the escapement locks again.
The importance of the escapement in the history of technology is that it was the key invention that made the all-mechanical clock possible. This development in 13th century Europe initiated a change in timekeeping methods from continuous processes, such as the flow of water in water clocks, to repetitive oscillatory processes, such as the swing of pendulums, which could yield more accuracy.
However, none of these were true mechanical escapements, since they still depended on the flow of liquid through an orifice to measure time. For example, in Su Song's clock, water flowed into a container on a pivot. The escapement's role was to tip the container over each time it filled up, thus advancing the clock's wheels each time an equal quantity of water was measured out. The development of mechanical clocks, though, depended on the invention of an escapement which would allow a clock's movement to be controlled by an oscillating weight. Unlike the continuous flow of water in the Chinese device, the medieval escapement was characterized by a regular, repeating sequence of discrete actions and the capability of self-reversing action:
Both techniques used escapements, but these have only the name in common. The Chinese one worked intermittently; the European, in discrete but continuous beats. Both systems used gravity as the prime mover, but the action was very different. In the mechanical clock, the falling weight exerted a continuous and even force on the train, which the escapement alternately held back and released at a rhythm constrained by the controller. Ingeniously, the very force that turned the scape wheel then slowed it and pushed it part of the way back . . . In other words, a unidirectional force produced a self-reversing action— about one step back for three steps forward. In the Chinese timekeeper, however, the force exerted varied, the weight in each successive bucket building until sufficient to tip the release and lift the stop that held the wheel in place. This allowed the wheel to turn some ten degrees and bring the next bucket under the stream of water while the stop fell back . . . In the Chinese clock, then unidirectional force produced unidirectional motion.
Actually, the earliest description of an escapement, in Richard of Wallingford's 1327 manuscript Tractatus Horologii Astronomici on the clock he built at the Abbey of St. Albans, was not a verge, but a variation called a 'strob' escapement. It 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.
However the verge was the standard escapement used in every other early clock and watch, and remained the only escapement for 400 years. Its friction and recoil limited its performance, but the accuracy of these 'verge and foliot' clocks was more limited by their early foliot type balance wheels, which because they lacked a balance spring had no natural 'beat', so there was not much incentive to improve the escapement.
The great leap in accuracy resulting from the invention of the pendulum and balance spring around 1657, which made the timekeeping elements in both watches and clocks harmonic oscillators, focused attention on the errors of the escapement, and more accurate escapements soon superseded the verge. The next two centuries, the 'golden age' of mechanical horology, saw the invention of perhaps 300 escapement designs, although only about 10 stood the test of time and were widely used. These are described individually below.
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, oxidation, etc., 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; examples include the grasshopper escapement of John Harrison in the 18th century, and the co-axial escapement of Daniels in the 20th century. This may avoid the need for lubrication in the escapement (though that doesn't affect the requirement for lubrication of other parts of the gear train).
The crucial element in escapement design is to give just enough energy to the pendulum in order to keep it swinging, and to interfere with the free swinging of the pendulum as little as is possible. As the lubrication of the escapement ages, friction will increase, and less power will be transferred to the timing device (for example, the pendulum). If the timing device is a pendulum, this means the pendulum will swing a shorter and shorter arc. Contrary to popular opinion, the time taken for a pendulum swing is not constant regardless of the size of the swing; the swing time changes with the size of the swing. Therefore, a dirty escapement will cause inaccuracy because the arc of the pendulum swing becomes shorter (the clock will speed up). To minimize this effect, pendulum swings are kept as small as possible.
Wristwatches, and smaller clocks, do not use pendulums as the timing device. Instead, they use balance-springs; fine springs connected to a metal "balance wheel" (imagine a bicycle wheel without the tire). The balance wheel spins back and forward; a good Swiss watch has a frequency of 4 Hz (or 8 beats). Faster speeds are used in some watches. The balance-spring must also be temperature neutral. Very sophisticated alloys are used; 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 spinning. Also, the same lubrication problem occurs over time; the watch will lose accuracy (typically it will speed up) when the escapement lubrication starts failing.
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 balance mechanism. The 'tourbillon' was invented to minimize this: the balance spring is put in a cage which rotates (typically once a minute), smoothing gravitational distortions. This very clever and sophisticated clock-work is a prized 'complication' in wrist-watches, even though the natural movement of the wearer tends to smooth gravitational influences much more than for a pocketwatch.
The most accurate mechanical clocks ever made are those by W. H. Shortt in 1921 and the Littlemore Clock built by noted archaeologist E. T. Hall. In Hall's paper, he reports an error of 3 parts in 109 measured over 100 days (an error of about 0.02 seconds over that period). The article compares that with a number of other precision clocks; the next best reported accuracy is that of the Shortt clock, a factor of 10 less accurate (30 parts in 109). 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.
The earliest escapement in Europe (from about 1275) is the verge escapement, also known as the crown-wheel-and-verge escapement. It pre-dates the pendulum and was originally controlled by a foliot, a horizontal bar with a weight at each end. A vertical shaft (verge) is attached to the middle of the foliot and carries two small plates (pallets) sticking out like flags from a flag pole. One pallet is near the top of the verge and one near the bottom and looking end-on down the verge the pallets are a little over ninety degrees apart. The escape wheel is shaped somewhat like a crown and turns about a horizontal axis. As the wheel tries to turn, one tooth of the wheel pushes against the upper pallet and starts the foliot moving. As the tooth pushes past the upper pallet, the lower pallet swings into the path of the escape wheel. The momentum of the moving foliot pushes the escape wheel backwards but eventually the system comes to rest. It is now the turn of the lower pallet to push the foliot and so on. The system has no natural frequency of oscillation - it is simply force pushing inertia around.
The next stage of development was to use the same idea but attach it to a pendulum. The axis of the verge became horizontal, one half of the foliot disappeared and the crown wheel rotated about a vertical axis. On a much smaller scale the same escapement was used for watches with a balance wheel and spring replacing the pendulum. John Harrison's first marine chronometer used a heavily-modified verge escapement and demonstrated that the verge could be capable of good timekeeping.
Invented around 1660 by Robert Hooke, the anchor quickly superseded the verge to become the standard escapement used in pendulum clocks through the 19th century. Its advantage was that it reduced the wide pendulum swing angles of the verge to 3°-6°, making the pendulum isochronous, and allowing the use of longer, slower moving pendulums, which used less energy. It is responsible for the long narrow shape of most pendulum clocks, particularly grandfather clocks.
It consists of an escape wheel with pointed, backward slanted teeth, and an 'anchor' shaped piece pivoted above it which rocks from side to side, attached to the pendulum. The anchor has slanted pallets on the arms which alternately catch on the teeth of the escape wheel, receiving impulses. Mechanically its operation has similarities 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 clocks gears, and inaccuracy. These problems were eliminated in the deadbeat escapement, which slowly replaced the anchor in precision clocks.
The deadbeat escapement was an improvement of the anchor escapement first made by Thomas Tompion to a design by Richard Towneley in 1675 although it is often credited to Tompion's successor George Graham who popularized it in 1715. In the anchor escapement the swing of the pendulum pushes the escape wheel backward during part of its cycle. This 'recoil' disturbs the motion of the pendulum, causing inaccuracy, and reverses the direction of the gear train, causing backlash and introducing high loads into the system, leading to friction and wear. The main advantage of the deadbeat is that it eliminated recoil.
In the deadbeat, the pallets have a second curved "locking" face on them, concentric about the pivot the anchor turns on. 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. This was the first escapement to separate the locking and impulse actions of the escapement. The deadbeat was first used in precision regulator clocks, but due to greater accuracy superseded the anchor in the 1800s. It is used in almost all modern pendulum clocks.
The lever escapement has been used by the vast majority of watches since the 1800s. It is accurate and reasonably easy to manufacture. It is a self-starting escapement, so if the watch is shaken so that the balance wheel stops, it will automatically start again. There are several forms of the lever escapement. The original form was the rack lever, in which the lever and the balance wheel were always in contact via a gear-like connection. Later, it was realized that all the teeth from the gears could be removed except one, and this created the detached lever escapement. Not only was the detached escapement easier to make and simpler, it was substantially more accurate. The lever could be configured so that it was at a right angle to the escape wheel, which was the common preference for British watchmakers. Or, the lever could be configured to be inline with the balance wheel and the escape wheel, which was the common preference for Swiss and American watchmakers. Finally, in the days of the "dollar watches", a very primitive form of the lever escapement was used, called the "pin-pallet escapement".
The detent or chronometer escapement was most commonly used on marine chronometers, although some precision watches during the 1800s also used it. It was invented in 1783 by Thomas Earnshaw (John Arnold created a variant, patented in 1779, but the Earnshaw design was the one generally adopted) and used until mechanical chronometers became obsolete in the 1970s. They could be made far more accurate than lever escapements, due to the virtual absence of sliding friction between the escape tooth and the pallet during impulse. Yet they were more fragile, were not self-starting, and harder to manufacture in volume. Unlike the lever escapement, the impulse to the balance occurs at every other swing. Usually a balance making 4 swings per second with a helical balance spring was used.
The cylinder escapement was an improvement over the verge escapement in terms of time keeping abilities, but it was not easy to manufacture, was somewhat fragile and the escape wheel was in constant contact with the balance which caused it to need regular cleaning due to wear. It was commonly used by the French watchmakers, although the Swiss also used it quite a bit.
The duplex watch escapement was invented by Robert Hooke around 1700, improved by Jean Baptiste Dutertre and Pierre Le Roy, and put in final form by Thomas Tryer, who patented it in 1782. It was used in quality English pocketwatches from about 1790 to 1860, and in the Waterbury, a cheap American 'everyman's' watch, during 1880-1898. 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. 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 (CW), and the process repeats. During the CW swing, the impulse tooth falls momentarily into the ruby roller notch again, but isn't released.
The duplex is a frictional rest escapement; the balance is never totally free from the escapement because of the tooth resting against the roller. 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. The duplex is capable of accuracy at least equal to the lever escapement, and perhaps approaching the chronometer. However it lost favor to the lever; 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 CW swing, it can't get started again.
Since it is the slave pendulum that releases the gravity lever, this synchronization is vital to the functioning of the clock. The slave clock is set to run slightly slow and the re-set circuit for the gravity arm activates a pivoted arm which just engages with the tip of a blade spring on the pendulum of the slave clock. If the slave clock has lost too much time its blade spring pushes against the arm and this accelerates the clock. The amount of this gain is such that the blade spring doesn't engage on the next cycle but does on the next again. This form of clock became a standard for use in observatories, and was the first clock capable of detecting small variations in the speed of Earth's rotation.