Escapement
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The escapement drives the pendulum in a pendulum clock, usually from a gear train. The gear train is powered to provide energy into the pendulum, typically using springs or weights. Without the escapement the system would simply unwind continuously, but the escapement makes this motion periodic, controlled by the pendulum. The pendulum moves the escapement back and forth, and makes 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. An escapement is also found in a mechanical watch, powering and regulated by a balance wheel and hairspring instead of a pendulum.
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[edit] History
The first escapement (of verge escapement type) purportedly originated in China with the Buddhist monk Yi Xing, who along with government official Liang Ling-zan applied its use in 723 (or 725) to the workings of a water-powered celestial globe. The Song Dynasty era official and horologist Su Song of the 11th century dually applied an escapement device for his astronomical clock-tower of medieval Kaifeng. The predecessor to the first escapement mechanism in Europe is still of unknown origin (thought to have been derived from a device of Villard de Honnecourt in 1237), yet the general accepted date for its separate invention is 1275 (with noted advancements in European clockworks between 1280 and 1320).
[edit] Reliability
The reliability of an escapement depends on the quality of workmanship and the level of maintenance given. A poorly maintained escapement will cause problems. The function of the escapement is to transmit controlled quantities of energy into the gear train at regular intervals (for example, to allow the main-spring of a wristwatch to drive the hands of the watch). The escapement must transfer enough power into the timing device so that it keeps running. If the timing device is a pendulum, then the escapement must provide a small kick each swing; for a wrist watch which uses a balance spring, the escapement must keep the balance-spring winding and unwinding. For most of clock-making history, these requirements have involved considerable friction in the escapement. In the animation above, the teeth of the wheel are powered by the clock's power source, and they actually slide against the ratchet teeth before they lock; this is the friction which kicks the pendulum but which requires lubrication. In a modern escapement, the teeth of the ratchet are very hard and very polished stone, but lubrication is still essential. The ticking sound of a wrist watch is the sound of the balance-spring gear and escapement teeth locking into with each other (eight impacts a second in a typical high-quality Swiss watch). If the lubrication fails (through age-related thinning or dust) then the escapement can suffer terrible damage and metal parts will require replacement. The increased reliability of modern watches is due primarily to the higher quality oils used to lubricate the escapement. Lubricant lifetimes can be greater than five years in a high-quality watch.
Daniels' 20th century innovation, the co-axial escapement, is notable primarily because it avoids the sliding friction of the anchor escapement. While this makes the escapement more accurate, the true breakthrough is in reliability due to the greatly reduced (or even totally avoided) lubrication requirements.
[edit] Accuracy
The accuracy of a mechanical clock is dependent on the accuracy of the timing device. If this is a pendulum, then the period of swing of the pendulum determines the accuracy. If the pendulum rod is made of metal it will expand and contract with heat, shortening or lengthening the pendulum; this changes the time taken for a swing. Special alloys are used in expensive pendulum-based clocks to minimise this distortion. Pendulum swings also vary according to how big the arc is; highly-accurate pendulum-based clocks have very small arcs. Pendulum-based clocks can achieve outstanding accuracy. Even into the 20th century, pendulum-based clocks were reference time pieces in laboratories, although at sea the natural motion of the vessel severely impairs the accuracy of a pendulum. Escapements play a big part in accuracy as well. The precise point in the pendulum's travel at which impulse is supplied, will determine 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 because pushing a pendulum when it's moving towards mid-swing makes it gain while pushing it while its 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. See Rawlings' The Science of Clocks.
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 minimise 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 tyre). 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 minimise 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.
[edit] Types
Many escapements have been designed and developed over the years. Today of course atomic clocks and GPS satellite receivers have replaced mechanical clocks for precision timekeeping purposes, and the continued development of more precise mechanical clocks has become a little-known curiosity. Perhaps the most accurate mechanical clocks ever made are those made in Russia by Feodosii Mikhailovich Fedchenko in the 1950s and later by the noted archaeologist Edward Hall in the UK. An obituary of Professor Hall that appeared in the Independent newspaper on 16 August 2001 says: "In later life he constructed at his house what he boasted was the most accurate pendulum clock in the world. Sunk in 18 tons of concrete, it varied some 0.01 seconds in three months and was, he said, 'the biggest waste of time anyone has conceived'."[1]
The following are some notable escapements:
[edit] Verge escapement
The earliest escapement (in China, 8th century, 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 chronometer used a heavily-modified verge escapement and demonstrated that the verge could be capable of good timekeeping.
[edit] Anchor escapement
In England, the anchor escapement largely superseded the verge, because the angle through which the pendulum needed to swing was very much reduced. This allowed the use of longer pendulums and saw the introduction of the longcase or grandfather clock. In France however the verge escapement continued to be used with its geometry modified to accommodate a smaller arc of operation. The teeth of an anchor escape wheel project radially from the edge of the wheel as with any ordinary gear wheel. Above the wheel are the anchor shaped pallets (rather like those in the animation at the top of this page, but upside down).
[edit] Deadbeat escapement
A clock with a deadbeat escapement was made by Thomas Tompion in 1675 although it was left to Tompion's successor George Graham, to make it widely known. It was an improved version of the anchor escapement. A pendulum continues to swing even after the teeth have locked, and with the verge and the anchor, this reverses the direction of the gear train. The traditional form of gears in clocks only works well going forwards so the recoil introduces high loads into the system, leading to friction and wear.
In Graham's escapement the pallets are curved about the same axis that they turn on: there is no recoil, so the locking face of the pallets provide no impulse. The impulse is provided by putting an angled plane surface on the end of the pallet so that as the escape wheel is released its tooth pushes along this wedge, impulsing the pendulum. This was the first escapement to separate the locking and impulse actions of the escapement. The escapement was adopted widely for precision and high-quality clocks and led to a number of later escapements which share its lack of recoil See Rawlings. The Science of Clocks
[edit] Lever escapement
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".
[edit] Chronometer escapement
The chronometer escapement was, as the name implies, most commonly used on marine chronometers. They could be made to be far more accurate than lever escapements, but they were more fragile, were not self-starting, and harder to manufacture.
[edit] Cylinder escapement
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.
[edit] Co-axial escapement
Invented and patented by George Daniels, the co-axial escapement is complicated but through a combination of interlocks avoids the sliding friction of the anchor escapement. It is a very important innovation in mechanical watchmaking in the sense that it solves the long-standing friction problem, although the accuracy and reliability gained still do not come close to electronic movements. Because mechanical watches no longer sell because of their accuracy or reliability as timepieces, the main watchmakers had little interest in investing in the tooling required, although finally Omega decided to implement this technology. While low-friction escapements existed already, they were too large for small "movements" (as clock-work is referred to).
[edit] Grasshopper escapement
A rare but interesting mechanical escapement is John Harrison's grasshopper escapement. In this escapement, the pendulum is driven by two hinged arms (pallets). As the pendulum swings, the end of one arm catches on the escape wheel and drives it slightly backwards; this releases the other arm which moves out of the way to allow the escape wheel to pass. When the pendulum swings back again, the other arm catches the wheel, pushes it back and releases the first arm and so on. The grasshopper escapement is more difficult to manufacture than other escapements and is something of a rarity. Grasshopper escapements made by Harrison in the 18th century are still operating. Most escapements wear far more quickly, and waste far more energy. In the bearings of his clocks he used Lignum vitae, a wood which is very hard, and is self lubricating.
[edit] Gravity escapement
A gravity escapement uses a small weight or a weak spring to give an impulse directly to the pendulum. The earliest form consisted of two arms which were pivoted very close to the suspension spring of the pendulum with one arm on each side of the pendulum. Each arm carried a small dead beat pallet with an angled plane leading to it. When the pendulum lifted one arm far enough its pallet would release the escape wheel. Almost immediately another tooth on the escape wheel would start to slide up the angle face on the other arm thereby lifting the arm. It would reach the pallet and stop. The other arm meanwhile was still in contact with pendulum and coming down again to a point lower than it had started from. This lowering of the arm provides the impulse to the pendulum. 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 turret clocks and has recently been perfected in the inertial-detached gravity escapement invented by James Arnfield. This frees the pendulum from any part in unlocking the clock train; all it does is lift a gravity arm and then later on part company from it at a lower point.
[edit] Electromechanical escapements
In the late 19th century, electromechanical escapements were developed. In these, a switch or phototube turned an electromagnet on for a brief section of the pendulum's swing. These are amongst some of the best escapements known. On some clocks the pulse of electricity that drove the pendulum would also drive a plunger to move the gear train.
[edit] Hipp clock
In the middle of the 19th century Matthias Hipp invented an ingenious switch for a clock which was impulsed electro-magnetically. The pendulum drove a ratchet wheel via a pawl on the pendulum rod and the ratchet wheel drove the rest of the clock train to indicate the time. The pendulum was not impulsed on every swing or even at a set interval of time. It was only impulsed when its arc of swing had decayed below a certain level. As well as the counting pawl, the pendulum also carried a small vane, pivoted at the top, which was completely free to swing. It was placed so that it dragged across a triangular polished block with a vee-groove in the top of it. When the arc of swing of the pendulum was large enough, the vane crossed the groove and swung free on the other side. If the arc was too small then the vane never left the far side of the groove and, when the pendulum swung back it pushed the block strongly downwards. The block carried a contact which completed the circuit to the electromagnet which impulsed the pendulum. The pendulum was only impulsed as it required it.
[edit] Free pendulum clock
In the 20th century William Harrison Shortt invented a free pendulum clock, patented in September of 1921 and manufactured by the Synchronome Company, with an accuracy of one hundredth of a second per day. In this system the time keeping "master" pendulum, whose rod is made from a special steel alloy with 36% nickel called Invar whose length does not change with temperature, swings as free of external influence as possible sealed in a vacuum chamber and does no work. It is in mechanical contact with its escapement for only a fraction of a second every 30 seconds. A secondary "slave" pendulum turns a ratchet, which triggers an electromagnet every thirty seconds. This electromagnet releases a gravity lever onto the escapement above the master pendulum. A fraction of a second later, the motion of the master pendulum releases the gravity lever to fall farther. In the process, the gravity lever gives a tiny impulse to the master pendulum, which keeps that pendulum swinging. The gravity lever falls onto a pair of contacts, completing a circuit that does several things:
- energizes a second electromagnet to raise the gravity lever above the master pendulum to its top position,
- sends a pulse to activate one or more clock dials, and
- sends a pulse to a synchronizing mechanism that keeps the slave pendulum in step with the master pendulum.
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.
[edit] Seiko electromechanical balance spring
In 2005 Seiko announced a new electro-mechanical movement. The movement is fundamentally mechanical, but uses passive electronics to control the speed of the balance wheel. Essentially, the balance wheel is controlled by treating it as an electrical generator, and by varying the load the generator drives. This means a very low-friction escapement, with quartz accuracy. It also means that for the first time for a wristwatch, the hands of the watch move completely smoothly, rather than in the eight steps per second of a purely mechanical premium watch or the one step per second of many quartz watches. Seiko claims the movement was under development for thirty years. The powersource driving the watch is a spring; it retains most of the features of a mechanical movement, although the mechanical movement is no longer responsible for the accuracy of the watch.
[edit] See also
[edit] External links
- Mark Headrick's horology page, with animated pictures of many escapements.
- American Watchmakers-Clockmakers Institute
- Federation of the Swiss Watch Industry
- Daniels coaxial escapement (sketch and accuracy review)
- Watch and Clock Escapements. A Complete Study in Theory and Practice of the Lever, Cylinder and Chronometer Escapements, Together with a Brief Account of the Origin and Evolution of the Escapement in Horology. Published in 1904, from Project Gutenberg
- [2] Website of the collector who owns Shortt Clock Number 6, with close-up pictures of the mechanism
- [3] US Patent number 5140565, issued 1992-03-23, for a novel type of pendulum clock
- [4] Obituary of Professor Edward Hall, 'The Independent' 16 August 2001
