Under the Loupe/The Hairspring
The Hairspring is the heart of a watch. It is the Hairspring, its physical properties, its size, thickness, length, and its form, that ultimately determine how well or how poorly a given movement will keep time.
The outer end of the Hairspring is fixed into a Stud and its inner end into a Collet. In older watches this Stud was fixed in place and the beat of the watch had to be adjusted by moving the length of the spring carried within stud hole or by adjusting its position relative to the balance at the Collet. In most modern watches the Stud is mobile and facilitates fine adjustment with far greater ease.
Types of Hairsprings
In vintage watches, the hairspring is often made of steel wire, ribbon-shaped, drawn hard, and coiled up into a close spiral. Modern hairsprings come in a variety of different alloys, designed to compliment the physical properties of the Balance with which they are used to overcome errors introduced by changes in temperature. They are further engineered to be both chemically and physically stable, resilient, and anti-magnetic.
Steel hairsprings require the use of split, bimetallic balances to compensate for changes in temperature. This type of balance was outfitted with a series of screws around its perimeter, which could be adjusted to correct for errors in timekeeping across a wide range of changes in temperature.
The discovery of Invar and Elinvar alloys about the turn of the 20th century, by Charles Edouard Guillaume, laid the groundwork for a new generation of hairsprings that possessed a near-zero coefficient of thermal expansion, were anti-magnetic, and resistant to corrosion.
Guillaume's progress towards the ideal hairspring was further perfected in the 1930s through the work of Dr. Straumann at Fabriques d'Assortiments Réunis (FAR), with the development of the Nivarox alloy. Broadly speaking, in addition to the basic elements of Invar, Nivarox uses high amounts of the element Cobalt along with higher traces of Chromium. The result is a hairspring that is wear-resistant, anti-magnetic, resistant to corrosion, and has a near-zero coefficient of thermal expansion. Coupled with a Glucydur Balance wheel this type of hairspring made the need for compensated balances almost obsolete.
New developments in hairspring technology at the turn of the 2nd millennium have brought the time-keeping potential of mechanical timepieces to even greater levels of accuracy. Research by the CSEM into Deep Reactive Ion Etching technology to produce Silicon hairsprings, Gideon Levingston's Carbontime hairspring, and other developments by Ulysse Nardin, Patek Philippe, A. Lange & Söhne, and Rolex have redefined the horizons of precision mechanical timekeeping.
The stud may be a small, grooved cylinder, triangle, or rectangle made of brass or any other metal, fixed into the Balance cock, Balance bridge or Hairspring stud carrier by means of a small screw. In some older, English Full plate watches, the stud may be fixed into the plate. Or it may come in the form of a small square of brass pushed tight into & held by friction in the balance cock or bridge, as in many old Geneva-made watches. A steel stud screwed to the plate or the balance cock or bridge may also be used, as in the best English lever, American, and Swiss watches of the early 20th century.
In most watches prior to the 21st century, the Collet was made of a small brass circle turned to fit the balance staff friction tight, and split to facilitate the necessary tension. Another form of collet, used in the best English levers of yesteryear, is that of a circle of steel, hardened and tempered, made to accurately fit the staff. It had two flats filed upon it, making it nearly oblong. Such a collet fit more truly, and was not as liable to become damaged; it also allowed the hairspring to be trued in the centre more easily.
Mass produced watches of the late 20th and early 21st century employed new technologies which allowed for the hairspring to be glued or laser-welded to a cross-like, steel collet with greater precision than was previously possible.
The strength of a hairspring depends on the type, thickness, and width of the hairspring used. These properties, in conjunction with the Active length of a hairspring, will determine the "beat" or "train" count of a watch. Most modern watches have beat counts of 18,000, 21,600, 28,800, or 36,000 beats per hour. Older watches, particularly pocket watches, are typically outfitted with 18,000, 16,200, or 14,400 trains; that is, that their gear trains are toothed to require that number of beats in one hour.
The beat or train count of a watch can be ascertained by multiplying the numbers of teeth in the centre, third, fourth, and escape wheels, and then dividing that value by the product of the number of teeth on the third, fourth, and escape wheel pinions. Twice this result is the number of beats per hour necessary for the going train.
It will be noticed that in nearly all trains the fourth wheel makes one revolution per minute. Furthermore, it may be noticed that when the fourth wheel has ten times as many teeth as the escape wheel pinion, the train is 18,000; when it is twelve times, the number of beats per hour is 21,600; when it is sixteen times, the number of beats per hour is is 28,800; and when it is two times that of an 18,000bph train, the resulting number of beats per hour is also double, at 36,000. It may also be noted that all of these values are multiples of 3,600, which itself is a multiple of 60. More specifically, it is the square of 60 (sixty minutes in an hour times sixty seconds in a minute), which is why the fourth wheel is often used as a convenient means of keeping record of the seconds.
The overcoil hairspring, which comes in a variety of forms and is most often referred to as a Bréguet hairspring or Bréguet overcoil, is an ideal form of hairspring with which near perfect timekeeping can be achieved. Specifically, unlike a normal, curbed, flat hairspring, the overcoil form allows the spring to breathe concentrically, thus paving the way for perfect Isochronism. Since its realization by Bréguet, nearly 200 years ago, the overcoil hairspring has undergone numerous alterations and improvements through great study, trial, error, and success, by other great watchmakers such as Philips and Grossman. Before the advent of Silicium based hairsprings, this type of spring was considered - and is still considered by many - to be the best form of hairspring for any watch. Helical or cylindrical springs, are used in marine chronometers and some pocket watches, but have not been deemed to perform any better than a single overcoil Bréguet hairspring.
Overcoil hairsprings are made from flat springs by the workman who springs the watch. They cannot be obtained for order as some flat hairsprings can, however it is sometimes possible for the common watchmaker to obtain one to meet a repair as part of a Balance-complete. Most overcoil springs today are made from Nivarox. Older pocket watches, with compensated balances are normally outfitted with tempered and blued steel hairsprings of overcoil form. Sometimes springs of Palladium wire are also used, especially in earlier, non-magnetic watches, as Palladium does not rust and cannot be magnetized.