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.
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
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.
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.
3345, Watches Australia.