DUPLEX ESCAPEMENT
Duplex escapement (2Hz)
Speed 1:3
Duplex escapement (2Hz)
Normal speed
Adapted from a pendulum clock escapement by Jean-Baptiste Dutertre, the duplex escapement reached its final functional design around 1750, thanks to Pierre Le Roy.
It takes its name from the double profile of its escape wheel, which features two distinct sets of teeth, each fulfilling a specific role in the unlocking and impulse phases of energy transmission.
Over the course of a century, the duplex escapement underwent numerous improvements aimed at enhancing precision and lowering production costs, particularly through the stamping of escape wheels. One of the best-known variants, the Duplex-Jacot escapement, was developed by Charles-Edouard Jacot. This version delivers one impulse for every four alternations (i.e. three lost beats). When combined with a balance oscillating at 14,400 A/h (or 4 alternations per second), it enables a deadbeat seconds display without requiring an additional complication.
A contemporary of the cylinder escapement, the duplex escapement enjoyed considerable success before ultimately being superseded by the Swiss lever escapement.
The duplex escapement, as described here, is a single-beat escapement, as it delivers only one impulse to the balance wheel per oscillation, and a frictional rest escapement, since a locking tooth of the escape wheel remains continuously in contact with the roller while the balance wheel performs its supplementary arc.
The balance wheel staff carries a notched cylinder, usually made of ruby, called the locking roller, which is fixed to the balance wheel staff. Above the locking roller is the impulse roller, pressed onto the balance wheel staff. This impulse roller is fitted with an impulse pin (or lifting pin) made of steel or, in higher-end constructions, ruby.
On the periphery of the escape wheel is a row of pointed teeth called locking teeth, while on the wheel body (rim) is another row of trapezoidal teeth called impulse teeth. Depending on the construction, the escape wheel may have between 12 and 15 teeth of each type, always in equal numbers.
Figure 1
The operation of the duplex escapement can be divided into four phases, which occur in a fixed cycle at each oscillation of the regulating organ. These four phases are:
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Locking
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Unlocking (small lift)
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Impulse (large lift)
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Silent beat (lost beat)
Below is a detailed description of the action of each component during these four phases:
1. Locking
While the balance wheel performs its supplementary arc, a locking tooth rests against the locking roller (Figure 2).
Figure 2
2. Unlocking (small lift)
As the balance wheel completes its descending supplementary arc, the locking tooth resting against the periphery of the locking roller enters the notch of the locking roller, thereby releasing the escape wheel (Figure 3).
Figure 3
3. Impulse (large lift)
The escape wheel catches up with the impulse roller. An impulse tooth of the escape wheel comes into contact with the impulse pin carried by the table (Figure 4).
Figure 4
As soon as the impulse phase is completed, the next locking tooth of the escape wheel comes to rest against the circumference of the locking roller while the balance wheel performs its ascending supplementary arc (Figure 5).
Figure 5
4. Silent beat (lost beat)
During this vibration, the balance wheel receives no impulse. The impulse pin on the impulse roller passes between two impulse teeth without any contact. The locking tooth remains in contact with the locking roller. As the notch passes the locking tooth, the escape wheel makes a brief twitch (a slight forward and backward motion) (Figure 6).
Figure 6
Note:
For a complete understanding of the operation of the duplex escapement, please refer to the two animations shown at the top of this page.
Advantages
The duplex escapement offers several notable advantages:
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Excellent rate stability of the balance due to the clear separation between the impulse and locking phases.
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Reduced friction compared to cylinder or verge escapements.
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Relative mechanical simplicity, with few moving components in interaction.
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Lower production cost, as the escape wheel could be manufactured by stamping.
These qualities made it particularly well suited to high-precision pocket watches prior to the widespread adoption of the Swiss lever escapement.
Disadvantages
Despite its technical merits, the duplex escapement also presents several limitations:
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Mechanical fragility, especially in the escape wheel’s double-profile design, whose locking teeth are particularly delicate.
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Shock sensitivity, making it ill-suited to watches worn in dynamic conditions.
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Unidirectional impulse, requiring highly accurate adjustment to maintain balanced oscillations of the balance wheel.
These shortcomings contributed to its gradual decline in favour of more reliable, low-friction detached escapements.