Conical vs. Square-Shouldered Pivots – What Are They?
Perhaps one of most confusing aspects of ordering replacement parts is the use of escape wheels with different styles of pivots which includes some with 2 different types of pivot. Not only was this done with escape wheels but manufactures did this with pallet fork arbors, 3rd and 4th wheels as well.
In watchmaking, the pivot refers to the slender axle end of a wheel that turns in a jewel bearing. A square-shouldered (cylindrical) pivot has an abrupt shoulder where the thin pivot meets the thicker arbor, whereas a conical pivot tapers to a point (like a tiny cone) instead of an abrupt shoulder. A conical pivot is typically paired with a cap jewel (a flat end-stone) in addition to the usual pierced hole jewel. This is because the conical pivot has no shoulder to limit its travel; the flat cap jewel serves to define the end shake (axial play) by contacting the pivot tip. In contrast, a square-shouldered pivot usually operates with just a hole jewel; the pivot’s shoulder itself limits end shake by butting against the inner face of the jewel or plate. Square (cylindrical) pivots have a shoulder and only need one jewel per side, while conical pivots require a cap jewel + hole jewel on that side to function properly.
Technical Reasons for Using a Conical Pivot on One Side
Watchmakers sometimes gave the escape wheel one conical pivot (with a cap jewel) and one traditional pivot for several technical benefits:
- Reduced Friction: A conical pivot with a cap jewel greatly reduces friction at that bearing. Instead of a wide shoulder rubbing a flat surface, the tiny pivot tip contacts the cap jewel, creating a point contact with minimal sliding. In vertical positions, the pivot only touches the thin edges of the hole jewel, again minimizing drag. Overall, a conical pivot bearing has less contact area than a shouldered pivot, improving efficiency in the high-speed escape wheel. This can improve amplitude and timekeeping, especially in positions, by eliminating some of the “shoulder drag” that a cylindrical pivot can have.
- Tighter End shake Control: Because the cap jewel physically limits the pivot’s endplay, the end shake can be set very precisely for the escape wheel. The conical pivot design “uses the cap jewel to determine end-shake”. This precise end shake can ensure the escape wheel stays aligned with the pallet fork for optimal lock and drop. It prevents the escape wheel from “flopping” excessively as you may have observed when assembling a watch where the cap jewel was left out. In short, adding a cap jewel and conical pivot can provide more consistent geometry and reduce variability in the escapement action.
- Reduced Wear and Heat Buildup: Less friction means less wear on the pivots and jewels over time. The small contact point at a conical pivot reduces frictional heating and abrasion of lubricant. Additionally, because the conical pivot doesn’t have a sharp corner, there’s a smoother interface – oil can form a meniscus more readily under the pivot tip, maintaining lubrication. The result is potentially longer-lasting lubrication and lower risk of a pivot cutting into its jewel due to friction.
- Greater Pivot Strength: The gradual taper of a conical pivot avoids the stress concentration that exists at the sharp shoulder of a square pivot. This can “strengthen the pivot against breakage”, which was important historically before modern shock absorbers. In a sudden impact (dropping the watch), a square-shouldered escape pivot might snap at the shoulder, whereas a conical pivot’s smooth transition distributes stress and may survive better. Note that the balance staff – the most impact-prone part – was the first to adopt conical pivots for exactly this reason. Applying the same idea to the escape wheel pivot could add durability.
- Consistent Performance in Positions: By having a cap jewel on one side, the escape wheel enjoys some benefits comparable to a fully capped bearing (though only on one end). In the common dial-up or dial-down positions, the escape wheel will rest on one of its pivots. If the conical pivot is on the downward-facing side, the wheel’s weight is borne on the cap jewel at a single point, keeping friction low and consistent. This can improve the watch’s rate consistency between face-up and face-down positions. (If the straight/shouldered pivot is downward instead, then dial-up/down friction is as normal on that side – see next section on configuration.)
Manufacturing Advantages
Aside from performance, there were practical manufacturing motives for this half-conical design:
- Incremental Jeweling & “Full Jewel” Marketing: In the mid-20th century, 17 jewels became the de-facto standard of a fully jeweled watch. Some movements originally had one fewer jewel (for example, one of the center wheel pivots might run in a bushing, giving 16 jewels). Rather than re-engineer the entire wheel train, manufacturers could add a single cap jewel to the escape wheel to bump the count to 17. Indeed, it’s noted that “to hit the coveted 17 ‘full’ jeweling, a cap jewel is added somewhere… Thus many iterations of escape wheels in the past with a conical pivot on one side”. The escape wheel was a logical place to add a jewel because it yields a real friction reduction benefit (unlike, say, jeweling a slow-moving center wheel). This approach was cost-effective: it required changing the escape wheel pivot on one end to conical and adding a cap jewel and setting, while leaving the rest of the movement unchanged.
- Modular Variations: Using one conical pivot made it easy to produce jewel-count variants of a movement. The base movement could use a straight-pivot escape wheel for a lower jewel count, or a conical-pivot version with an extra jewel for a deluxe model. Bulova, for example, followed this approach. In their parts lists, the same caliber often had two escape wheel versions: one labeled “straight pivots (S/S)” for the standard model, and one with “conical (or S/C) pivots” for the higher-jeweled model. For instance, Bulova’s 10BZAC (a 17-jewel caliber) uses an escape wheel with straight pivots, while the 8AH (21-jewel) or 10CRACD (30-jewel) movements use escape wheels with a conical pivot design to accommodate the extra cap jewels. This modularity simplified manufacturing and inventory – the gear train and plates remained the same, with just the escape wheel and jewel settings differing.
- Service Considerations: In service, a conical pivot escape wheel can be identified and handled appropriately (e.g. one must remove and replace the cap jewel when cleaning). Watchmakers have long been aware of these variations – “watchmakers have always ordered train wheels by specifying straight or conical pivots where applicable”. It’s crucial that the correct type is used: if a conical-pivot wheel were installed in a movement without its cap jewel, the pivot could sink too far into the jewel hole and bind when the plates are tightened. Conversely, a straight pivot wheel in a cap-jewel movement would have excess end shake. Thus, having one conical and one shouldered pivot was a deliberate design matched to the jewel configuration. When properly matched, these parts don’t complicate servicing much – though the watchmaker must mind the tiny cap jewel and be aware of pivot differences.
Typical Configurations – Which Side Had the Conical Pivot?
Historically, when only one end of the escape wheel was given a conical pivot and cap jewel, it was usually the upper (train-bridge) side. The train side (the side of the movement opposite the dial) was seen as better to equip with a cap jewel because it’s visible when removing the case back.
The Bestfit catalog shows these typical layouts- figure 1.
This escape wheel chart shows the nomenclature used for different pivot layouts. For context, the top of the movement is the side with the balance and the bottom is the dial side of the movement. The first letter of the two letter code represents the upper pivot type and the second letter represents the bottom pivot. The “S” stands for straight pivot and the “C” stands for conical pivot.
You can see that #702 has two conical pivots so in this case the top and bottom jewel setting would have end stones. #703 would have a regular jewel on top and a setting with an end stone on the bottom of the movement. # 704 would have a setting with an end stone on the top and a regular jewel on the bottom and #705 would have two regular jewels one the top and bottom.
There are exceptions: some designs placed the cap jewel on the lower (dial) side instead. For instance, the A. Schild (AS) caliber 1012 had an escape wheel version listed as “S/C – upper pivot Straight, lower pivot Conical”, indicating the conical pivot (with cap jewel) was on the lower side even though the vast majority of mid-century wristwatch makers put the extra cap jewels on the top side of the train for marketing purposes.
Here is a lot of AS 1012’s that were being sold on eBay-Figure 2.
In the first movement (A), you see the end stone of the escape wheel. If you look at the bottom side of figure 3-D, notice that the escape wheel has a regular jewel. This configuration would be a C/S.
In figure 2-B, the escape wheel has a standard jewel on the top and and end stone on the bottom, figure 3-E. This configuration would be S/C.
In figure 2-C, notice that not only does the escape wheel have an end stone, but so does the 3rd and 4th wheel. The configuration for the escape wheel on this movement is C/C.
Same movement all with different jewel counts and different type of escape wheels.
Implications: The side on which the conical pivot is placed will see the biggest friction reduction when that side faces downward (gravity loading that pivot). So if the conical pivot is on the upper/train side, the watch will have lower friction in the dial-down position (since the escape wheel then rests on the upper pivot and cap). If the watch is dial-up, that upper conical pivot is unloaded (hanging upward) and the lower straight pivot carries the weight – in that position the escape wheel behaves like a regular one, with the shoulder pivot against the lower jewel. Thus, the improvement is asymmetric: one face-up/down position enjoys the full benefit. Manufacturers likely judged this acceptable, as watches spend time in various orientations; the cap jewel still prevents wear in one orientation and boosts the jewel count. In any case, adding even one cap jewel on the escape wheel gave some performance gain and allowed finer adjustment of end shake on that end. Have the end stone on the top of the movement also made more logical sense because the dial up position is more important than the dial down position when the watch is worn.
Notable Examples
- Bulova (1930s–1950s): Bulova produced numerous calibers in both 17-jewel and higher-jeweled variants. The 21-jewel variants often added cap jewels to the escape wheel (and sometimes the fourth wheel or pallet pivots). For example, the Bulova 8AH was made in a 17j version with a straight escape pivot and a 21j version where the escape wheel had a conical pivot and cap jewels added. Bulova’s material catalogs explicitly differentiate escape wheels by “S/S” (both pivots straight) versus “S/C” or “C/S” (one straight, one conical). The conical pivot was typically on the upper side in these Bulova movements. This design was also used in Bulova’s higher jewel Count models in the 1950s (often marketed under names like “23 Jewels” to emphasize the extra jeweling).
- Other Swiss Makes: Ebauche makers like Adolph Schild (AS) and ETA often provided alternate escape wheels and jewel sets for different jewel counts. As noted, AS 1012 had an option for a lower conical pivot. Jaeger-LeCoultre’s calibers from the 1940s (e.g. the JLC 480) also had part listings for escape wheels with conical pivots (one part number for straight, another for conical). These were usually used in higher-grade versions of the movement that included extra cap jewels on the escape wheel. Some high-end watches even capped both ends of the escape wheel (making both pivots conical, requiring four jewels total for the escape wheel pivots) – for example, certain 23-jewel chronometer-grade movements did this to minimize friction in all positions. However, using one cap jewel/pivot was a more economical middle step that still yielded benefit.
- Watchmaking References: So why was this design was used? The consensus is that it was a functional improvement and a practical way to increase jewel counts. It eliminates the need for the pivot’s shoulder to rub on a jewel, thereby reducing friction and tightening end shake. I would also point out the strength advantage of a conical shape, Nearly all very thin pivots (under 0.15 mm) are made conical to avoid breakage. The escape wheel pivots are indeed quite thin, so this was a wise precaution in the era before shock protection became widespread.
In summary, historical Swiss lever escapements with one conical and one square pivot on the escape wheel were engineered for smoother, more efficient running and to facilitate incremental jeweling upgrades. Typically the conical pivot (and its cap jewel) was on the bridge side, giving the escape wheel a low-friction point of contact and precise end shake control on that end. This design reduced friction and wear in at least one orientation, strengthened the delicate pivot, and allowed manufacturers like Bulova to offer “up-jeweled” models with minimal redesign. It’s a clever compromise that provided real technical advantages while also satisfying the marketing demand for higher jewel counts in the mid-20th-century watch industry.