Every once in a while, a watch mechanism comes in for service that features cap jewels that differ in thickness between the upper and lower anti-shock settings, which guard the delicate pivots of the balance staff. This is more common with older watch calibres than with modern ones, which tend to keep these component sizes the same to help minimize production, assembly, and service costs. Whether new calibre or old, though, if the thickness of the cap jewels differ, it’s important to take note, as placing a cap jewel that’s too thick or too thin into the wrong setting could spell disaster. The design engineers who develop these tiny, ticking, mechanical wonders have a reason behind every detail of the watch mechanisms’ construction, down to the most minute details, and the thickness they choose to make the cap jewels is no exception.
The role of a cap jewel is to minimize friction. Plain and simple. In the case of a typical jeweled or metal bushing, the watch wheel that rides in that bushing is subjected to friction both around the circumference of the pivot and on the shoulder of arbor that the pivot is cut from. In a cap jewel setting, however, the only points of contact where friction can occur are at the very tip of the pivot and around the circumference of the pivot. In addition, to help minimize friction around the circumference of the pivot, the jeweled bushing beneath most cap jewel settings typically features a hole with rounded edges, as opposed to straight, flat edges. Furthermore, cap jewels are typically only used on very thin pivots, so the overall surface area that can be subjected to the opposing force of friction is greatly reduced as well, both at the tip of the pivot and around its circumference.
The role of a cap jewel in an anti-shock setting, such as those manufactured by KIF and Incabloc, is still to minimize friction. However, the cap jewel isn’t fixed permanently in place, but rather is held in a mobile chaton by means of a spring. The purpose being to help prevent the delicate pivots it protects from breaking when the watch incurs a strong hit or hard fall. When the watch is hit hard, the tension on the spring, which holds the cap jewel in place, is light enough that the pivot can push the cap jewel up, slightly, out of the chaton, allowing the shoulder of the pivot to take the brunt of the blow as it comes into contact with the bottom of the chaton. After the shock has passed, the spring quickly returns everything to its intended position.
In order for the cap jewel in an anti-shock setting to fulfill its purpose effectively, the jewel needs to be thick enough that it doesn’t shatter under the force of an impact to the watch while also being thin enough to provide enough space for the pivot to displace it far enough to allow the shoulder of the pivot to take the full force of a blow. In the case of the lower cap jewel, which is fitted in the mainplate, there is often very little room for the jewel to be displaced between the dial and it’s usual place of rest. This is especially true of older calibre designs that were tweaked only slightly by manufacturers to accommodate anti-shock jewels in place of their predecessors, the fixed cap jewel. To this end, in such cases, the cap jewels used in anti-shock settings on the dial side balance a much finer line between ruggedness and function than their counterparts on the backside of a movement. The space between the mainplate and the dial of the watch is limited. A cap jewel can only travel so far before it collides with the underside of the dial, but the cap jewel has to travel far enough to help absorb to force of a sharp blow. Thus, in order to allow enough travel to effectively absorb a shock before coming into contact with the dial, these cap jewels are purposefully made thinner. The cap jewels in the balance cock or bridge, on the other hand, have more room to travel and can, therefore, err on the thicker and more rugged side.
While we are only talking fractions of a millimeter in difference between the upper and lower cap jewel – in such cases where any difference exists – this microscopic difference goes a long way when the pivots they’re helping to protect are a mere 0.09mm in diameter. So the next time you find yourself assembling a watch movement with differing cap jewels, keep this one simple design insight in mind: thin cap jewels were tailored for the dial side.