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THE REMONTOIR

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1. Definition and Principle

The Remontoir is a mechanism interposed between the Motor organ (power source) and the Escapement. Its role is to deliver a constant and regular force to the Regulating organ. The Mainspring, as it gradually unwinds, provides a decreasing force to the finishing train. This variation in torque affects the Isochronism and degrades precision. The Remontoir neutralizes this effect by interposing a small secondary spring (in the shape of a spiral) in the train. This secondary spring is periodically rewound by the mainspring, then released at regular intervals. Thus, the Escapement always receives the force of a fully wound secondary spring. The Remontoir is classified among precision complications.

2. Position within the family of precision complications

Precision complications aim to improve the chronometric rate of a Movement. The Remontoir shares this goal with other devices, notably the Tourbillon. However, the two mechanisms act on distinct sources of error. The Tourbillon compensates for the influence of gravity on the Regulating organ in vertical position. The Remontoir, on the other hand, neutralizes the variation in force of the Mainspring. Since these two sources of error are independent, a movement can combine both complications. It is important to distinguish the remontoir from the constant-force escapement, which acts on the same cause but is the subject of a separate article.

3. Operating principle

The principle of the remontoir is based on the interposition of a secondary spring acting on the Escape wheel. This spring is generally located between the last wheel of the train and the Escape wheel. The Train (wheel train) rewinds this secondary spring at regular and defined intervals. Between two successive rewinds, the secondary spring alone drives the Escapement via a Cam. Since this spring is fully wound at the start of each interval, the transmitted force is constant. The release and rewinding mechanism must be frequent enough to remain effective. Indeed, if the interval is too long, the secondary spring partially relaxes and reintroduces a variation in force. The regularity of this interval is therefore the key parameter of the mechanism.

4. The Remontoir spring

The winding spring is one of the central elements of the mechanism. It is a small spring, generally spiral-shaped. Its Stiffness (spring rate) and free Length are calculated to store a precisely defined amount of energy. A well-calibrated spring will deliver a constant force over the entire Amplitude of its release. The design of this spring is delicate, as it must combine flexibility and adequate Stiffness (spring rate). On one hand, it must be easily wound by the motor Train (wheel train). On the other hand, it must provide sufficient force to drive the Escapement with regularity. These conflicting requirements require a rigorous choice of Material and geometry.

5. The Vibration-based remontoir

The Vibration-based remontoir is the fastest variant of the Remontoir. It rewinds the secondary spring at each Vibration of the Balance, that is at every half-oscillation. Thus, for a Balance beating at 4 Hz, rewinding occurs eight times per second. This high Frequency ensures that the secondary spring practically never relaxes. The force delivered to the Escapement is therefore extremely stable. In return, the mechanical complexity of this type of remontoir is considerable. The locking and release mechanism must operate with absolute precision at a very high rate. This variant is the most demanding to design, produce and regulate.

6. The seconds remontoir

The seconds remontoir operates at longer intervals than the Vibration remontoir. The seconds remontoir releases and rewinds the secondary spring once per second. For this type of remontoir, the admissible force variation is smaller, which makes a longer interval acceptable. In both cases, the periodic rewinding is often visible as a jump of theHand seconds.

7. Chronometric advantages

The main advantage of the Remontoir is to make the Movement’s rate independent of the state of charge of the Mainspring. Without this mechanism, the torque variation between a fully wound and an empty Barrel can reach several tens of percent. This variation alters the Amplitude of the Balance, and, due to the non-isochronism of the Hairspring (balance spring), causes a variation in Frequency. With the Remontoir, the Amplitude remains stable and the Daily rate is much more consistent. This advantage is especially notable in movements with a long Power reserve, fitted with a single Barrel. Indeed, the longer the reserve, the more extended over time the Barrel’s force variation becomes.

8. Design Constraints

The design of a Remontoir imposes several specific technical constraints. The friction inherent to the Remontoir must not cancel out the benefits of constant force. This is why the parts of the Remontoir are carefully polished and their bearings are executed with precision. Furthermore, the secondary spring must be carefully calibrated so as not to reintroduce a variation in force. Finally, adding this mechanism increases the number of parts in the Movement and makes Assembly and Regulation / Adjustment more complex.

9. Evolution and Modern Applications

The principle of the Remontoir has been known since the 17th century, in various forms. It was widely used in large precision clocks and observatory Regulators. For the wristwatch, its realization is more delicate due to the reduced dimensions of the Movement. Since the 1990s, several high-end watchmaking manufactures have nevertheless taken up this challenge. The rise of Silicon facilitated the miniaturization of the mechanism, enabling springs that were lighter and more consistent in their production. These developments made it possible to apply a complication once reserved for precision instruments to modern portable watches. Although rare, the Remontoir has thus become a highly prized precision complication.