What are Jewel Bearings in a watch and why are they crucial?

“I’ve heard that there are some rubies on your watch? Must very precious, right? Wait, where are they? I can’t see them.” And when looking at a movement for the first time, people often ask what are the little red dots on their watch’s movement or why watches all mention the number of jewels? If you’re a watch collector, this conversation should be familiar. After looking at the evolution of the critical part that is the escapement, A Technical Perspective will today focus on an often underestimated part of a watch, even if its role is vital to a flawless run: jewel bearings.

A mechanical movement is composed of moving parts, such as gears, wheels and pinions. These parts are rotating on pivots (a vertical axis crossing the wheel). However, some has to consider that, for example, the fourth wheel of a watch makes a full circle on its axis once per minutes, meaning that it does 1440 full rotations a day or 525 600 full rotations a year. And this is not even the fastest wheel of a watch… Thus, you have to imagine the forces applied on the axis of such a wheel, even if it’s small and light. This is why it is harmful to insert the axis of such a wheel directly in the main-plate of the movement – considering the metal vs. metal frictions. Even with modern lubricants, it will slow the movement of the wheels down – ruining the accuracy of the watch – and at a certain time, wheels will simply block. Jewel bearings are the solution.

Jewels (rubies) are used as bearings to reduce friction at critical points, to improve the accuracy and durability of watches. One of the hardest materials after diamond, ruby (a corundum) do not wear – almost. As a consequence, jewels can be made very smooth which allow low friction with metal pivots. Last, they have a very good temperature stability and are acid-resistant.

Cross section of jewels bearings in mechanical watches

A bit of history

Before the invention of jewels, metal pivots and bearings rubbing against each other was a major concern for watchmakers, leading to severe damages in movements, specifically with oil aging over time.

At the beginning of the 18^th century, Swiss mathematician and astronomer Nicolas Fatio de Duillier invented a technique to drill hard stones with precision, in order to use them as bearings in mechanical watches. Originally, the jewels were made with garnets (which were much cheaper and available than rubies), sapphire, rubies or even diamonds (which have been used by Abraham Louis Breguet). Yet, the amount of work needed to drill such stones limited their use to high quality watches only. To the point that, over the years, clients associated jewels (and the number of jewels) with high grade watches, all the more that the movement of complicated watches tend to require more of them. For instance, to import watch movements to the USA, these are required to be marked to show “in words the number of jewels, if any, serving a mechanical purpose as frictional bearings”.

Jewel, synthetic ruby ©Pierhor

In the late 18^th century, French chemist Pierre Victor Louis de Verneuil invented the flame fusion process, allowing to produce artificial corundum (sapphire or rubies). This inventions allows to manufacture large quantities of artificial stones, more homogeneous than natural ones, making jewel bearings much cheaper.

Anti-shock systems

Anti-shock system by KIF Parechoc

The pivots and jewel bearings of the balance wheel are especially fragile and were often damaged under impact. They were a major cause of repair. With anti-shock systems, the jewels are mounted on springs allowing them to shift in their setting, in order to absorb radial or axial shocks. The widely used Incabloc, Kif or Etachoc are the more common shock protection systems for Swiss movement manufacturers.

Thanks to Francois Durafourg from Pierhor for his help.