BARREL

The barrel is a set of several components, usually four in number: the barrel drum, the cover of the barrel drum, the barrel arbor, and the mainspring.

The role of the barrel is to store the energy accumulated during the winding (manual or automatic) of the watch and gradually release it to the movement.

To assemble the barrel, the relaxed spring is placed inside the drum. The mainspring is a spiral-wound steel blade capable of developing considerable force given its size. The barrel-arbor is then placed in the centre of the drum. The inner coil of the mainspring is attached to the arbor by a hook system (on the arbor) and notches. The assembly is then closed with the cover that fits onto the barrel drum.

The role of the barrel arbor is twofold. During winding, it allows the spring to be wound around its circumference (and thus at the centre of the drum) to arm the spring and give the watch its full autonomy. During watch winding, the winding mechanism drives the ratchet wheel, which is fitted onto a machined square at one end of the shaft. The barrel arbor also serves as the axis and pivot for the barrel, allowing it to rotate freely. The mainspring is thus held to the barrel arbor by a hook system, and its other end is fixed (by a notch and a bridge system) to the inner wall of the barrel drum, thereby putting the spring under tension.

The force exerted by the spring on the drum wall, once armed, tends to rotate the entire barrel around its axis when the spring seeks to unwind. Teeth are milled on the larger diameter of the drum. This gearing allows the centre pinion to be driven, consequently followed by the entire kinematic chain of the movement.

Once fully wound, the spring reaches its maximum tension. If the winding mechanism is then forced, it inevitably causes the mainspring to break. Thus, as early as the 17th century, mechanisms for limiting winding (finger, Maltese cross, etc.) were introduced. It’s a system that blocks the winding mechanism just before the mainspring reaches its maximum tension.

Unfortunately, such a setup does not apply to watches equipped with an automatic winding system. The mechanism of the automatic system is unable to detect the over-tensioning of the mainspring or to stop supplying it when the spring reaches its maximum tension. To prevent the mainspring from breaking in such a situation, automatic watch springs are equipped with a sliding bridge. This bridge, located at the outer end of the mainspring, hooks into a notch in the inner wall of the barrel drum. When the spring slightly exceeds its maximum tension, the bridge, through its construction and the shape of the drum’s notch, comes out of its notch and slides along the inner wall of the drum before hooking into the next notch (several notches are made on the inner wall of the drum). Once the watch is fully wound, the movements of the wearer of an automatic watch keep the spring at maximum tension. The barrel thus distributes maximum and constant energy, ensuring a better quality of regulation.

To mitigate the risk of mainspring breakage, many manual-winding watches today are equipped with an “automatic” type spring, allowing endless winding without any danger.

While the manufacture of the drum, its cover, and the barrel arbor does not pose great difficulty, the production of springs requires very specific equipment and expertise. Their production is systematically subcontracted, and only a few manufacturers share this market.

The quality of a barrel essentially depends on its spring. This spring should have optimal efficiency (minimizing energy loss for an increased power reserve) and develop a force as constant as possible throughout its deployment (precision of the movement). Precise calculations, incorporating the nature of its alloy, dimensions, and thermal treatments, allow to production of a high-performance and suitable barrel.

To increase the power reserve and/or achieve a more constant force throughout the entire work of the watch, the number of barrels can be multiplied. Similar to an electrical circuit, a series arrangement of barrels will increase the power reserve, while their parallel operation will distribute more constant energy throughout the duration of their work.

Due to the robustness and comfortable dimensions of its arbor, the barrel easily lends itself to a suspended construction, meaning that the barrel is fixed to the mainplate by a single end of its axis. The use of a barrel bridge is therefore no longer necessary and often allows for minimizing the thickness of the movement and improving its aesthetics. The recent development of micro-ceramic ball bearings allows this type of construction to achieve quite satisfactory efficiency.