YOKE SPRING
A yoke spring is an integral part of both the winding and time-setting mechanisms. It serves to maintain continuous contact between the yoke and the setting lever. Depending on the watch’s quality, it can be a strip-spring cut from a mass of steel or a simple wire spring.
The yoke and its spring emerged in 1847 with Antoine Le Coultre’s invention of the setting lever winding mechanism, where the setting lever was a central element. Since then, it has facilitated the switching of the crown control between the winding and time-setting mechanisms, at a minimum.
The major advancements made to Le Coultre’s invention have aimed to multiply the functions adjustable by the crown.
The watchmaker begins the artisanal production of the yoke spring by engraving its profile using a tracer on the surface of a steel plate slightly thicker than the finished component. Next, the watchmaker marks the various drill points (for screws and feet). Then, the contour of the spring is delicately cut using a piercing saw. The sides of the spring are then filed to bring the spring to its final thickness and in absolute accordance with its form. The heel and blade of the spring are then beveled and polished using customary tools (files, buff sticks, polishing). These operations are particularly delicate due to the length, fineness, and shape of the spring’s blade. According to traditional craftsmanship, the watchmaker then draws out the lines on the sides of the piece (satin finish) and the tiny flat portion that remains on the upper surface of the spring.
Heat treatments and subsequent blanching operations are also particularly delicate due to the length and fineness of the spring.
The electrical discharge process is used to optimize production costs for a single piece or a small series of components. Implementation costs and machine processing time remain relatively low and suitable for small production volumes. Additionally, this technology allows for the cutting of complex and fine profiles without exerting any mechanical stress on the component during its fabrication. Compared to the artisanal method, electrical discharge cutting offers considerable time-saving with positive impacts on the decoration stages. The end of the manufacturing and decoration process resembles the artisanal method or the required level of finishing.
For large-scale production, the investment in a stamping tool justifies manufacturing yoke lever springs at an optimal cost. The expanded outline of the profile is thus stamped directly into a strip of steel slightly thicker than the finished component by a stamping press. Manual steps, however, remain unavoidable. The yoke spring must be brought to its final dimensions (profile and thickness) while providing the desired finishes and decorations (satin finish, beveling, etc.).
On an industrial scale, and for a less refined quality than that obtained by the method described above, manufacturers commonly use a simple wire spring. This is a length and shape-adjusted steel wire that is sufficiently elastic to fulfil its spring function.