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| [[File:Alarm Clock Balance Wheel.jpg|thumb|220px|Balance wheel in a cheap 1950s alarm clock, the Apollo, by Lux Mfg. Co. showing the balance spring (1) and regulator (2)]]
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| [[File:Chinese movement escapement and jewels.jpg|right|thumb|Modern balance wheel, in a Chinese watch movement]]
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| }}
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| A '''balance wheel''' is the timekeeping device used in [[mechanical watch]]es and some [[clock]]s, analogous to the [[pendulum]] in a [[pendulum clock]]. It is a weighted wheel that rotates back and forth, being returned toward its center position by a spiral [[torsion spring]], the [[balance spring]] or hairspring. It is driven by the [[escapement]], which transforms the rotating motion of the watch [[Wheel train (horology)|gear train]] into impulses delivered to the balance wheel. Each swing of the wheel (called a 'tick' or 'beat') allows the gear train to advance a set amount, moving the hands forward. The balance wheel and hairspring together form a [[harmonic oscillator]], which due to [[resonance]] oscillates preferentially at a certain rate, its [[resonant frequency]] or 'beat', and resists oscillating at other rates. The combination of the mass of the balance wheel and the [[Elasticity (physics)|elasticity]] of the spring keep the time between each [[oscillation]] or ‘tick’ very constant, accounting for its near universal use as the timekeeper in mechanical watches to the present. From its invention in the 14th century until [[quartz clock|quartz]] movements became available in the 1970s, virtually every portable timekeeping device used some form of balance wheel.
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| ==Overview==
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| Until the 1980s balance wheels were used in [[Marine chronometer|chronometers]], [[bank vault]] [[time lock]]s, time [[fuze]]s for [[munition]]s, [[alarm clock]]s, kitchen [[timers]] and [[stopwatch]]es, but [[Quartz clock|quartz]] technology has taken over these applications, and the main remaining use is in quality mechanical [[watch]]es.
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| Modern (2007) watch balance wheels are usually made of [[Glucydur]], a low thermal expansion alloy of [[beryllium]], [[copper]] and [[iron]], with springs of a low thermal coefficient of elasticity alloy such as [[Nivarox]].<ref name="Odets">{{cite web
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| | last=Odets
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| | first=Walt
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| | year=2007
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| | title=The Balance Wheel of a Watch
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| | work=The Horologium
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| | publisher=TimeZone.com
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| | url=http://www.timezone.com/library/horologium/horologium0013
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| | accessdate=2007-06-16| archiveurl= http://web.archive.org/web/20070706233745/http://www.timezone.com/library/horologium/horologium0013| archivedate= 6 July 2007 <!--DASHBot-->| deadurl= no}}</ref> The two alloys are matched so their residual temperature responses cancel out, resulting in even lower temperature error. The wheels are smooth, to reduce air friction, and the pivots are supported on precision [[jewel bearing]]s. Older balance wheels used weight screws around the rim to adjust the poise (balance), but modern wheels are computer-poised at the factory, using a laser to burn a precise pit in the rim to make them balanced.<ref>{{cite web
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| | last=Odets
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| | first=Walt
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| | year=2005
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| | title=Balance Wheel Assembly
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| | work=Glossary of Watch Parts
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| | publisher=TimeZone Watch School
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| | url=http://www.timezonewatchschool.com/WatchSchool/Glossary/Glossary%20-%20Balance%20Assembly/glossary%20-%20balance%20assembly.shtml
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| | accessdate=2007-06-15}}</ref> Balance wheels rotate about 1½ turns with each swing, that is, about 270° to each side of their center equilibrium position. The rate of the balance wheel is adjusted with the '''regulator''', a lever with a narrow slit on the end through which the balance spring passes. This holds the part of the spring behind the slit stationary. Moving the lever slides the slit up and down the balance spring, changing its effective length, and thus the resonant vibration rate of the balance. Since the regulator interferes with the spring's action, chronometers and some precision watches have ‘free sprung’ balances with no regulator, such as the [[Gyromax]].<ref name="Odets" /> Their rate is adjusted by weight screws on the balance rim.
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| A balance's vibration rate is traditionally measured in '''beats''' (ticks) per hour, or BPH, although beats per second and [[Hz]] are also used. The length of a beat is one swing of the balance wheel, between reversals of direction, so there are two beats in a complete cycle. Balances in precision watches are designed with faster beats, because they are less affected by motions of the wrist.<ref>{{cite web
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| | last=Arnstein
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| | first=Walt
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| | year=2007
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| | title=Does faster mean more accurate?, TimeZone.com
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| | url=http://www.timezone.com/library/comarticles/comarticles0017
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| | accessdate=2007-06-15}}</ref> Watches made prior to the 1970s usually had a rate of 5 beats per second (18,000 BPH). Current watches have rates of 6 (21,600 BPH), 8 (28,800 BPH) and a few have 10 beats per second (36,000 BPH). During WWII, Elgin produced a very precise stopwatch that ran at 40 beats per second (144,000 BPH), earning it the nickname 'Jitterbug'.<ref>{{cite web
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| | last=Schlitt
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| | first=Wayne
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| | year=2002
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| | title=The Elgin Collector's Site
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| | url=http://elginwatches.org/databases/watch_codes.html
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| | accessdate=2007-06-20}}</ref> Audemars Piguet currently produces a movement that allows for a very high balance vibration of 12 beats/s (43,200 BPH).<ref>http://professionalwatches.com/2009/01/sihh_2009_jules_audemars_with.html</ref>
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| The precision of the best balance wheel watches on the wrist is around a few seconds per day. The most accurate balance wheel timepieces made were [[marine chronometer]]s, which by WWII had achieved accuracies of 0.1 second per day.<ref>{{cite encyclopedia | encyclopedia=Encyclopædia Britannica online| year=2007 | title=Marine Chronometer |publisher= Encyclopædia Britannica Inc. | url=http://www.britannica.com/ebc/article-9360740 | accessdate=2007-06-15}}</ref>
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| ==Period of oscillation==
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| A balance wheel's [[Frequency|period of oscillation]] ''T'' in seconds, the time required for one complete cycle (two beats), is determined by the wheel's [[moment of inertia]] ''I'' in kilogram-meter<sup>2</sup> and the stiffness ([[spring constant]]) of its [[balance spring]] ''κ'' in newton-meters per radian:
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| :<math>T = 2 \pi \sqrt{ \frac {I}{\kappa} } \,</math>
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| [[File:De Vick Clock Verge & Foliot.png|thumb|220px|Foliot from De Vick clock, built 1379, Paris]]
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| ==History==
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| [[File:Giovanni Di Dondi clock .png|thumb|left|Perhaps the earliest existing drawing of a balance wheel, in [[Giovanni de Dondi]]'s [[astronomical clock]], built 1364, Padua, Italy. The balance wheel (crown shape, top) had a beat of 2 seconds. Tracing of an [http://books.google.com/books?id=o8Nb5KLBxVQC&pg=PA106&ots=8eA4W86mAP&dq=balance+wheel&sig=HHahEKuxnZKFyh94vlzLJRoqaI0 illustration] from his 1364 clock treatise, ''Il Tractatus Astrarii''.]]
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| The balance wheel appeared with the first mechanical clocks, in 14th century Europe, but it seems unknown exactly when or where it was first used. It is an improved version of the [[Verge escapement|foliot]], an early inertial timekeeper consisting of a straight bar pivoted in the center with weights on the ends, which oscillates back and forth. The foliot weights could be slid in or out on the bar, to adjust the rate of the clock. The first clocks in northern Europe used foliots, while those in southern Europe used balance wheels.<ref>{{cite book | last=White | first=Lynn Jr. | title=Medieval Technology and Social Change | year=1966 | publisher=Oxford Press | isbn=978-0-19-500266-9}}, p.124</ref> As clocks were made smaller, first as [[bracket clock]]s and [[lantern clock]]s and then as the first large watches after 1500, balance wheels began to be used in place of foliots.<ref>{{cite book | last=Milham | first= Willis I. | title=Time and Timekeepers | year=1945 | publisher=MacMillan | location=New York | isbn=0-7808-0008-7}}, p.92</ref> Since more of its weight is located on the rim away from the axis, a balance wheel could have a larger [[moment of inertia]] than a foliot of the same size, and keep better time. The wheel shape also had less air resistance, and its geometry partly compensated for [[thermal expansion]] error due to temperature changes.<ref name="Headrick">{{cite journal | last=Headrick | first=Michael | year=2002 | title=Origin and Evolution of the Anchor Clock Escapement | periodical=Control Systems magazine, Inst. of Electrical and Electronic Engineers | volume=22 | issue=2 | url=http://www.geocities.com/mvhw/anchor.html | accessdate=2007-06-06|archiveurl=http://web.archive.org/web/20091025120920/http://geocities.com/mvhw/anchor.html|archivedate=2009-10-25}}</ref>
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| ===Addition of balance spring===
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| [[File:Balance Wheel in Early Watch Berthoud.png|thumb|Early balance wheel with spring in an 18th-century French watch]]
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| These early balance wheels were crude timekeepers because they lacked the other essential element: the [[balance spring]]. Early balance wheels rotated freely in each direction until the [[escapement]] pushed it back the other way. This made the timekeeping strongly dependent on the driving force, so the watch slowed down as the [[mainspring]] unwound.
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| A way forward opened when it was noticed that springy hog bristle curbs, added to limit the rotation of the wheel, increased its accuracy.<ref>{{cite book | last=Britten | first=Frederick J. | title=On the Springing and Adjusting of Watches | year=1898 | publisher=Spon & Chamberlain | location=New York | url=http://books.google.com/?id=1SgJAAAAIAAJ&pg=PA9 | accessdate=2008-04-16 }} p.9</ref><ref>{{cite book | last=Brearley | first=Harry C. | title=Time Telling through the Ages | year=1919 | publisher=Doubleday | location=New York | url=http://books.google.com/?id=UO3j49UJ17wC&pg=RA1-PA109 | accessdate=2008-04-16 }} p.108-109</ref> [[Robert Hooke]] first applied a metal spring to the balance in 1658 and [[Jean de Hautefeuille]] and [[Christiaan Huygens]] improved it to its present spiral form in 1674<ref name="Headrick" /><ref>Milham 1945, p.224</ref> The addition of the spring made the balance wheel a [[harmonic oscillator]], the basis of every modern [[clock]]. This means the wheel vibrated at a natural [[resonant frequency]] or ‘beat’ and resisted changes in its vibration rate caused by friction or other disturbances. This crucial innovation greatly increased the accuracy of watches, from several hours per day<ref>Milham 1945, p.226</ref> to perhaps 10 minutes per day,<ref name="NIST">{{cite web | year=2002 | title=A Revolution in Timekeeping, part 3 | work=A Walk Through Time | publisher=NIST (National Inst. of Standards and Technology) | url=http://physics.nist.gov/GenInt/Time/revol.html | accessdate=2007-06-06 |archiveurl = http://web.archive.org/web/20070528005441/http://physics.nist.gov/GenInt/Time/revol.html <!-- Bot retrieved archive --> |archivedate = 2007-05-28}}</ref> changing them from expensive novelties into useful timekeepers.
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| ===Temperature error===
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| After the balance spring was added, a major remaining source of inaccuracy was the effect of temperature changes. Early watches had balance springs made of plain steel, and balances of brass or steel, and these were affected by temperature to the extent that the rate of even the earliest balance spring watches watch was noticeably affected.
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| An increase in temperature increases the dimensions of the balance spring and the balance due to [[thermal expansion]]. The strength of a spring, the restoring force it produces in response to a deflection, is proportional to the its breadth and the cube of its thickness, and inversely proportional to its length. An increase in temperature would actually make a spring stronger if it affected only its physical dimensions. However, a much larger effect in a balance spring made of plain steel is that the [[Elasticity (physics)|elasticity]] of the spring's metal decreases significantly as the temperature increases, the net effect being that a plain steel spring becomes weaker with increasing temperature. An increase in temperature also increases diameter of a steel or brass balance wheel, increasing its rotational inertia, its [[moment of inertia]], making it harder for the balance spring to accelerate. The two effects of increasing temperature on physical dimensions of the spring and the balance, the strengthening of the balance spring and the increase in rotational inertia of the balance, have opposing effects and to an extent cancel each other.<ref name="Rawlings">A.L. Rawlings, Timothy Treffry, The Science of Clocks and Watches, Publisher: BHI, ISBN 0 9509621 3 9, Edition: 1993, 3rd enlarged and revised edition.</ref> The major effect of temperature which affects the rate of a watch is the weakening of the balance spring with increasing temperature.
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| In a watch that is not compensated for the effects of temperature the weaker spring takes longer to return the balance wheel back toward the center, so the ‘beat’ gets slower and the watch loses time. [[Ferdinand Berthoud]] found in 1773 that an ordinary brass balance and steel hairspring, subjected to a 60°F (33°C) temperature increase, loses 393 seconds (6 1/2 minutes) per day, of which 312 seconds is due to spring elasticity decrease.<ref>[http://books.google.com/books?id=1SgJAAAAIAAJ&pg=PA37 Britten 1898, p.37]</ref>
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| ===Temperature compensated balance wheels=== | |
| The need for an accurate clock for [[celestial navigation]] during sea voyages drove many advances in balance technology in 18th century Britain and France. Even a 1 second per day error in a [[marine chronometer]] could result in a 17-mile error in ship's position after a 2 month voyage. [[John Harrison]] was first to apply temperature compensation to a balance wheel in 1753, using a [[Bimetallic strip|bimetallic]] ‘compensation curb’ on the spring, in the first successful marine chronometers, H4 and H5. These achieved an accuracy of a fraction of a second per day,<ref name="NIST" /> but the compensation curb was not further used because of its complexity.
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| [[File:Pocket Watch Balance Wheel.jpg|left|thumb| Bimetallic temperature compensated balance wheel, from an early 1900s pocket watch. 17 mm dia. (1) Moving opposing pairs of weights closer to the ends of the arms increases temperature compensation. (2) Unscrewing pairs of weights near the spokes slows the oscillation rate. Adjusting a single weight changes the poise, or balance.]]
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| A simpler solution was devised around 1765 by [[Pierre Le Roy]], and improved by [[John Arnold (watchmaker)|John Arnold]], and [[Thomas Earnshaw]]: the ''Earnshaw'' or ''compensating'' balance wheel.<ref>Milham 1945, p.233</ref> The key was to make the balance wheel change size with temperature. If the balance could be made to shrink in diameter as it got warmer, the smaller [[moment of inertia]] would compensate for the weakening of the balance spring, keeping the period of oscillation the same.
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| To accomplish this, the outer rim of the balance was made of a ‘sandwich’ of two metals; a layer of steel on the inside fused to a layer of brass on the outside. Strips of this [[Bimetallic strip|bimetallic]] construction bend toward the steel side when they are warmed, because the thermal expansion of brass is greater than steel. The rim was cut open at two points next to the spokes of the wheel, so it resembled an S-shape (see figure) with two circular bimetallic ‘arms’. These wheels are sometimes referred to as "Z - balances". A temperature increase makes the arms bend inward toward the center of the wheel, and the shift of mass inward reduces the [[moment of inertia]] of the balance, compensating for the reduced torque produced by the weaker balance spring. The amount of compensation is adjusted by moveable weights on the arms. Marine chronometers with this type of balance had errors of only 3–4 seconds per day over a wide temperature range.<ref>{{cite book
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| | last=Glasgow
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| | first=David
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| | title=Watch and Clock Making
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| | year=1885
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| | publisher=Cassel & Co.
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| | location=London
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| | url=http://books.google.com/?id=9wUFAAAAQAAJ
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| | accessdate=2008-04-16 }} p.227</ref> By the 1870s compensated balances began to be used in watches.
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| ===Middle temperature error===
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| [[File:Early Chronometer Balance Wheels.png|thumb|right|500px|Marine chronometer balance wheels from the mid-1800s, with various 'auxiliary compensation' systems to reduce middle temperature error]]
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| The standard Earnshaw compensation balance dramatically reduced error due to temperature variations, but it didn't eliminate it. As first described by J. G. Ulrich, a compensated balance adjusted to keep correct time at a given low and high temperature will be a few seconds per day fast at intermediate temperatures.<ref>{{cite book | last=Gould |first=Rupert T. | title=The Marine Chronometer. Its History and Development | location=London | publisher=J. D. Potter | year=1923 | isbn=0-907462-05-7 }} p.176-177</ref> The reason is that the moment of inertia of the balance varies as the square of the radius of the compensation arms, and thus of the temperature. But the elasticity of the spring varies linearly with temperature.
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| To mitigate this problem, chronometer makers adopted various 'auxiliary compensation' schemes, which reduced error below 1 second per day. Most of the chronometers that came in first in the annual [[Greenwich Observatory]] trials between 1850 and 1914 were auxiliary compensation designs.<ref>Gould 1923, p.265-266</ref> Auxiliary compensation was never used in watches because of its complexity.
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| ===Better materials===
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| [[File:Benrus Watch Balance Wheel 2.jpg|thumb|Low temperature coefficient alloy balance and spring, in an [[ETA SA|ETA]] 1280 movement from a Benrus Co. watch made in the 1950s]]
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| The bimetallic compensated balance wheel was made obsolete in the early 20th century by advances in metallurgy. [[Charles Edouard Guillaume]] won a Nobel prize for the 1896 invention of [[Invar]], a nickel steel alloy with very low thermal expansion, and [[Elinvar]] ('''''El'''asticité '''invar'''iable'') an alloy whose elasticity is unchanged over a wide temperature range, for balance springs.<ref>Milham 1945, p.234</ref> A solid Invar balance with a spring of Elinvar was largely unaffected by temperature, so it replaced the difficult-to-adjust bimetallic balance. This led to a series of improved low temperature coefficient alloys for balances and springs.
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| Guillaume himself invented an alloy to compensate for middle temperature error by endowing it with a negative quadratic temperature coefficient. This alloy is a slight variation of invar. The quadratic coefficient is defined by its place in the equation of expansion of a material;<ref>Gould, p.201.</ref>
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| :<math> l_\theta = l_0 (1 + \alpha \theta + \beta \theta^2) \,</math>
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| :where;
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| :<math>\scriptstyle l_0</math> is the length of the sample at some reference temperature
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| :<math>\scriptstyle \theta</math> is the temperature above the reference
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| :<math>\scriptstyle l_\theta</math> is the length of the sample at temperature <math>\scriptstyle \theta</math>
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| :<math>\scriptstyle \alpha</math> is the linear coefficient of expansion
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| :<math>\scriptstyle \beta</math> is the quadratic coefficient of expansion
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| ==References==
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| * {{cite encyclopedia | encyclopedia=Encyclopædia Britannica online| year=2007 | title=Marine Chronometer |publisher= Encyclopædia Britannica Inc. | url=http://www.britannica.com/ebc/article-9360740 | accessdate=2007-06-15}}
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| * {{cite book | last=Britten | first=Frederick J. | title=On the Springing and Adjusting of Watches | year=1898 | publisher=Spon & Chamberlain | location=New York | url=http://books.google.com/?id=1SgJAAAAIAAJ&q=hog+bristle#search | accessdate=2008-04-20 }}. Has detailed account of development of balance spring.
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| * {{cite book | last=Brearley | first=Harry C. | title=Time Telling through the Ages | year=1919 | publisher=Doubleday | location=New York | url=http://books.google.com/?id=UO3j49UJ17wC&pg=RA1-PA109 | accessdate=2008-04-16 }}.
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| * {{cite book | last=Glasgow | first=David | title=Watch and Clock Making | year=1885 | publisher=Cassel & Co. | location=London | url=http://books.google.com/?id=9wUFAAAAQAAJ | accessdate=2008-04-16 }}. Detailed section on balance temperature error and auxiliary compensation.
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| * {{cite book | last=Gould |first=Rupert T. | title=The Marine Chronometer. Its History and Development | pages=176–177 | location=London | publisher=J. D. Potter | year=1923 | isbn=0-907462-05-7 }}
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| * {{cite journal | last=Headrick | first=Michael | year=2002 | title=Origin and Evolution of the Anchor Clock Escapement | periodical=Control Systems magazine, Inst. of Electrical and Electronic Engineers | volume=22 | issue=2 | url=http://www.geocities.com/mvhw/anchor.html | accessdate=2007-06-06|archiveurl=http://web.archive.org/web/20091025120920/http://geocities.com/mvhw/anchor.html|archivedate=2009-10-25}}. Good engineering overview of development of clock and watch escapements, focusing on sources of error.
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| * {{cite book | last=Milham | first= Willis I. | title=Time and Timekeepers | year=1945 | publisher=MacMillan | location=New York | isbn=0-7808-0008-7}}. Comprehensive 616p. book by astronomy professor, good account of origin of clock parts, but historical research dated. Long bibliography.
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| * {{cite web | last=Odets | first=Walt | year=2005 | title=Balance Wheel Assembly | work=Glossary of Watch Parts | publisher=TimeZone Watch School | url=http://www.timezonewatchschool.com/WatchSchool/Glossary/Glossary%20-%20Balance%20Assembly/glossary%20-%20balance%20assembly.shtml | accessdate=2007-06-15| archiveurl= http://web.archive.org/web/20070614163315/http://www.timezonewatchschool.com/WatchSchool/Glossary/Glossary%20-%20Balance%20Assembly/glossary%20-%20balance%20assembly.shtml| archivedate= 14 June 2007 <!--DASHBot-->| deadurl= no}}. Detailed illustrations of parts of a modern watch, on watch repair website
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| * {{cite web | last=Odets | first=Walt | year=2007 | title=The Balance Wheel of a Watch | work=The Horologium | publisher=TimeZone.com | url=http://www.timezone.com/library/horologium/horologium0013| accessdate=2007-06-15| archiveurl= http://web.archive.org/web/20070706233745/http://www.timezone.com/library/horologium/horologium0013| archivedate= 6 July 2007 <!--DASHBot-->| deadurl= no}}. Technical article on construction of watch balance wheels, starting with compensation balances, by a professional watchmaker, on watch repair website
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| ==External links==
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| *{{cite web|last=Choi|first=Fred|title=William Simcock Massey Type III pocket watch|date=2007-05-26|publisher=YouTube|url=http://www.youtube.com/watch?v=U2PlpbwdJBE|accessdate=2008-04-26}} Video of antique mid-19th century watch showing the balance wheel turning
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| *{{cite web | last=Costa | first=Alan | year=1998 | title=The History of Watches | publisher=Atmos Man | url=http://www.atmos-man.com/historyo.html | accessdate=2007-06-19}} History of watches, on commercial website.
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| *Oliver Mundy, [http://www.horologia.co.uk/watchcabinet.html The Watch Cabinet] Pictures of a private collection of antique watches from 1710 to 1908, showing many different varieties of balance wheel.
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| ==Footnotes==
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| {{reflist|2}}
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| {{DEFAULTSORT:Balance Wheel}}
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| [[Category:Timekeeping components]]
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