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balance staff

Mechanical watch

A mechanical watch is a watch that uses a non-electric mechanism to measure the passage of time. They are driven by a spring (called a mainspring) which must be wound periodically, and releases the energy to turn the clock's wheels as it unwinds. They keep time with a balance wheel, which oscillates back and forth at a constant rate, and make a 'ticking' sound when operating. Mechanical watches evolved in Europe in the 1600s from spring powered clocks, which appeared in the 1400s.

Mechanical watches are not as accurate as modern quartz watches and are generally more expensive. They are now kept more for their aesthetic qualities and as jewelry than for their timekeeping ability.

Mechanism

All mechanical watches have these five parts:

  • A mainspring that stores mechanical energy to power the watch.
  • A gear train, called the going train, which has the dual function of transmitting the force of the mainspring to the balance wheel, and adding up the swings of the balance wheel to get units of seconds, minutes and hours, etc. A separate part of the gear train called the keyless work allows the user to wind the mainspring and enables the hands to be moved to set the time.
  • A balance wheel which oscillates back and forth. Each swing of the balance wheel takes precisely the same amount of time. This is the timekeeping element in the watch.
  • An escapement mechanism which has the dual function of keeping the balance wheel vibrating by giving it an impulse each swing, and allowing the clock's gears to advance or 'escape' by a set amount with each swing. It is this part that produces the characteristic 'ticking' sound of the mechanical watch.
  • An indicating dial, usually a traditional clock face with rotating hands, to display the time in human-readable form.

Additional functions on a watch besides the basic timekeeping ones are traditionally called complications. Mechanical watches may have these complications:

  • Automatic winding or self-winding - in order to relieve the need to wind the watch, this device winds the watch's mainspring automatically using the natural motions of the wrist, with a rotating weight mechanism.
  • Calendar - displays the date, and often weekday, month, and year. Simple calendar watches don't account for the different lengths of the months, requiring the user to reset the date 5 times a year, but perpetual calendar watches account for this, and even leap years. An annual calendar doesn't make the leap year adjustment, so the date must be reset on March 1 every fourth year.
  • Alarm - a bell or buzzer that can be set to go off at a given time.
  • Chronograph - a watch with additional stopwatch functions. Buttons on the case start and stop the second hand and reset it to zero, and usually several subdials display the elapsed time in larger units.
  • Hacking feature - found on military watches, a mechanism that stops the second hand while the watch is being set. This enables watches to be synchronized to the precise second. This is now a very common feature on many watches.
  • Moon phase dial - shows the phase of the moon with a moon face on a rotating disk.
  • Wind indicator or power reserve indicator - mostly found on automatic watches, a subdial that shows how much power is left in the mainspring, usually in terms of hours left to run.
  • Repeater - a watch that chimes the hours audibly at the press of a button. This rare complication was originally used before artificial lighting to check what time it was in the dark. These complex mechanisms are now only found as novelties in extremely expensive luxury watches.
  • Tourbillon - this expensive feature was originally designed to make the watch more accurate, but is now simply a demonstration of watchmaking virtuosity. In an ordinary watch the balance wheel oscillates at different rates when the watch is in different positions, causing inaccuracy. In a tourbillon, the balance wheel is mounted in a rotating cage so it will experience all positions equally. The mechanism is usually exposed on the face to show it off.

How it works

The mechanical watch is a mature technology, and virtually all ordinary watch movements have the same parts and work the same way.

The spiral mainspring that powers the watch is inside a cylindrical barrel, with the outer end of the mainspring attached to the barrel. The barrel has gear teeth around the outside that turn the center wheel once per hour — this wheel has a shaft that goes through the dial. On the dial side the cannon pinion is attached with a friction fit (allowing it to slide when setting the hands) and the minute hand is attached to the cannon pinion. The cannon pinion drives a small 12-to-1 reduction gearing called the motion work that turns the hour wheel and hand once for every 12 revolutions of the minute hand.

The center wheel drives the third wheel, which in turn drives the fourth wheel. In watches with the second hand in a subdial on the face, the fourth wheel is geared to rotate once per minute, and the second hand is attached directly to this wheel. In watches with a center second hand, the fourth wheel turns a gear on a shaft projecting through the center of the minute hand shaft, which turns the second hand.

The fourth wheel also drives the escape wheel of the lever escapement. The escape wheel teeth alternately catch on two fingers called pallets on the arms of the pallet lever, which rocks back and forth. The other end of the lever has a fork which engages with an upright impulse pin on the balance wheel shaft. Each time the balance wheel swings, it moves the lever, which releases one tooth of the escape wheel, allowing the watch's wheels to advance by a fixed amount, moving the hands forward. As the escape wheel turns, its tooth pushes against the lever, which gives the balance wheel a brief push, keeping it swinging back and forth.

The balance wheel keeps time for the watch. It consists of a weighted wheel which rotates back and forth, which is returned toward its center position by a fine spiral spring, the balance spring. The mass of the balance wheel combines with the stiffness of the spring to precisely control the period of each swing or 'beat' of the wheel. Most watch balance wheels oscillate at 5, 6, 8, or 10 beats per second. In most watches there is a regulator lever on the balance spring which is used to adjust the rate of the watch. It has two curb pins which embrace the last turn of the spring, and can be slid up or down the spring to control its effective length.

A separate set of gears called the keyless work winds the mainspring when the crown is rotated, and when the crown is pulled out a short distance allow the hands to be turned to set the watch. The stem attached to the crown has a gear called the clutch or castle wheel, with two rings of teeth that project axially from the ends. When the stem is pushed in, the outer teeth turn the ratchet wheel on top of the mainspring barrel, which turns the shaft that the inner end of the mainspring is attached to, winding the mainspring tighter around the shaft. A springloaded pawl or click presses against the ratchet teeth, preventing the mainspring from unwinding. When the stem is pulled out, the inner teeth of the castle wheel engage with a gear which turns the minute wheel. When the crown is turned, the friction coupling of the cannon pinion allows the hands to be rotated.

Watch jewels

Jewel bearings were invented and introduced in watches by Nicolas Fatio (or Facio) de Duillier and Pierre and Jacob Debaufre around 1702 to reduce friction. Until the 20th century they were ground from tiny pieces of natural gems. Watches often had garnet, quartz, or even glass jewels; only top quality watches used sapphire, ruby, or diamond. In 1902, a process to grow artificial sapphire crystals was invented, making jewels much cheaper. Jewels in modern watches are all synthetic sapphire or (usually) ruby, made of corundum (Al2O3), one of the hardest substances known. The only difference between sapphire and ruby is that different impurities have been added to change the color; there is no difference in their properties as a bearing. The advantage of using jewels is that their ultrahard slick surface has a lower coefficient of friction with metal. The static coefficient of friction of steel-on-steel is 0.58, while that of sapphire-on-steel is 0.10-0.15.

Why they are used

Jewels serve two purposes in a watch. First, reduced friction can increase accuracy. Friction in the wheel train bearings and the escapement causes slight variations in the impulses applied to the balance wheel, causing variations in the rate of timekeeping. The low, predictable friction of jewel surfaces reduces these variations. Second, they can increase the life of the bearings. In unjeweled bearings, the pivots of the watch's wheels rotate in holes in the plates supporting the movement. The sideways force applied by the driving gear causes more pressure and friction on one side of the hole. In some of the wheels, the rotating shaft can eventually wear away the hole until it is oval shaped, and the watch stops.

Types

In the escapement, jewels are used for the parts that work by sliding friction:

  • Pallets - These are the angled rectangular surfaces on the lever that are pushed against by the teeth of the escape wheel. They are the main source of friction in a watch movement, and were one of the first sites to which jewels were applied.
  • Impulse pin - The off center pin on a disk on the balance staff which is pushed by the lever fork, to keep the balance wheel moving.

In bearings two different types are used:

  • Hole jewels - These are donut shaped sleeve bearings used to support the arbor (shaft) of most wheels.
  • Capstones or cap jewels - When the arbor of a wheel is in the vertical position, the shoulder of the arbor bears against the side of the hole jewel, increasing friction. This causes the rate of the watch to change when it is in different positions. So in bearings where friction is critical, such as the balance wheel pivots, flat capstones are added at each end of the arbor. When the arbor is in a vertical position, its rounded end bears against the surface of the capstone, lowering friction.

Where they are used

Where jewels are used in watches
7 jewel lever watch - has these jewels:

  • 1 impulse pin
  • 2 pallets
  • 2 balance staff bearings
  • 2 balance staff capstones

11 jewel watch - adds:

  • 2 lever bearings
  • 2 escape wheel bearings

15 jewel watch - adds:

  • 2 fourth wheel bearings
  • 2 third wheel bearings

17 jewel watch - adds:

  • 2 center wheel bearings

21 jewel watch - adds:

  • 2 lever capstones
  • 2 escape wheel capstones

23 jewel watch - adds:

  • 2 mainspring barrel bearings

Self winding watches add 4 or more in the winding mechanism, for a total of 25-27

The number of jewels used in watch movements increased over the last 150 years as jeweling grew less expensive and watches grew more accurate. The only bearings that really need to be jeweled in a watch are the ones in the going train - the gear train that transmits force from the mainspring barrel to the balance wheel - since only they are constantly under force from the mainspring. The wheels that turn the hands (the motion work) and the calendar wheels are not under load, while the ones that wind the mainspring (the keyless work) are used very seldom, so they don't get significant wear. Friction has the greatest effect in the wheels that move the fastest, so they benefit most from jewelling. So the first mechanism to be jeweled in watches was the balance wheel, followed by the escapement. As more jeweled bearings were added, they were applied to slower moving wheels, and jewelling progressed up the going train toward the barrel. A 17 jewel watch has every bearing from the balance wheel to the mainspring barrel jeweled, so it was considered a 'fully jeweled' watch. In quality watches, to minimize positional error, capstones were added to the lever and escape wheel bearings, making 21 jewels. Even the mainspring barrel arbor was sometimes jeweled, making the total 23. When self winding watches were introduced in the 1950s, several wheels in the automatic winding mechanism were jeweled, increasing the count to 25-27.

'Jewel inflation'

It is doubtful whether adding jewels in addition to the ones listed above is really useful in a watch. It doesn't increase accuracy, since the only wheels which have an effect on the balance wheel, those in the going train, are already jeweled. Marine chronometers, the most accurate portable timepieces, often have only 7 jewels. Nor does jewelling additional wheel bearings increase the useful life of the movement; as mentioned above most of the other wheels don't get enough wear to need them.

However, by the early 20th century watch movements had been standardized to the point that there was little difference between their mechanisms, besides quality of workmanship. So watch manufacturers made the number of jewels, one of the few metrics differentiating quality watches, a major advertising point, listing it prominently on the watch's face. Consumers, with little else to go on, learned to equate more jewels with more quality in a watch. Although initially this was a good measure of quality, it gave manufacturers an incentive to increase the jewel count.

Around the 1960s this 'jewel craze' reached ridiculous heights, and manufacturers made watches with 41, 53, 75, or even 100 jewels. Most of these additional jewels were totally nonfunctional; they never contacted moving parts, and were included just to increase the jewel count. For example the Waltham 100 jewel watch consisted of an ordinary 17 jewel movement, with 83 tiny pieces of ruby mounted around the automatic winding rotor. In 1974, the International Organization for Standardization (ISO) in collaboration with the Swiss watch industry standards organization NIHS (Normes de l'Industrie Horlogère Suisse) published a standard, ISO 1112, which prohibited manufacturers from including such nonfunctional jewels in the jewel counts in advertising and sales literature.

This put a stop to the use of totally nonfunctional jewels. However, some experts say manufacturers have continued to inflate the jewel count of their watches by 'upjeweling'; adding functional jeweled bearings to wheels that don't really need them, exploiting loopholes in ISO 1112. Examples given include adding capstones to third and fourth wheel bearings, jeweling minute wheel bearings, and automatic winding ratchet pawls. Arguably none of these additions adds to the accuracy or longevity of the watch.

History

The first spring-powered pocket watch was created by Peter Henlein in 1524. In 1556, Taqi al-Din created a spring-powered pocket watch that could measure time in minutes, by having three dials for the hours, degrees and minutes.

Until the quartz revolution of the 1960's, all watches were mechanical. Early watches were terribly imprecise; a good one could vary as much as 15 minutes in a day. Modern precision (a few seconds per day) was not attained by any watch until 1760, when John Harrison crated his marine chronometer. Mecahnical watches are powered by a mainspring. Because the mainspring provides an uneven source of power (its torque steadily decreases as the spring unwinds), watches from the early 1500s to the early 1800s featured a chain-driven fusee which served to regulate the torque output of the mainspring throughout its winding. Unfortunately, the fusees were very brittle, were very easy to break, and were the source many problems, especially inaccuracy of timekeeping when the fusee chain became loose or lost its velocity after the lack of maintenance.

As new kinds of escapements were created which served to better isolate the watch from its time source, the balance spring, watches could be built without a fusee and still be accurate.

Basically three types of escapements have been industrially used: "lever", "pin-lever", and "Roskopf", latter invented by Georges Frederic Roskopf for cheaper watches.

As manual-wound mechanical watches became less popular and less favored in the 1970s, watch design and industrialists came out with the Automatic Watch Movement. Whereas a mechanically-wound watch must be wound with the pendant or a levered setting, an Automatic watch does not require to be wound by the pendant, but by simply shaking the watch winds the watch automatically. The interior of an Automatic Watch houses a swivelling metal or brass "plate", that swivels on its axes when the watch is shaken horizontally.

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